remove readonly speclet (#46568)

* remove readonly speclet

The standard now includes the language for `readonly` instance members in `struct` types. See dotnet/csharpstandard#673.

Remove the feature speclet from the learn site.

* build error

* warnings, part 2

Author: BillWagner

.openpublishing.redirection.csharp.json

@@ -116,6 +116,10 @@
             "source_path_from_root": "/redirections/proposals/csharp-8.0/nested-stackalloc.md",
             "redirect_url": "/dotnet/csharp/language-reference/language-specification/structs#164127-stackalloc"
         },
+        {
+            "source_path_from_root": "/redirections/proposals/csharp-8.0/readonly-instance-members.md",
+            "redirect_url": "/dotnet/csharp/language-reference/language-specification/structs#16411-properties"
+        },
         {
             "source_path_from_root": "/redirections/proposals/csharp-9.0/nullable-reference-types-specification.md",
             "redirect_url": "/dotnet/csharp/language-reference/language-specification/types.md#893-nullable-reference-types"

docfx.json

@@ -80,6 +80,7 @@
                     "csharp-8.0/nullable-reference-types.md",
                     "csharp-8.0/nullable-reference-types-specification.md",
                     "csharp-8.0/nested-stackalloc.md",
+                    "csharp-8.0/readonly-instance-members.md",
                     "csharp-9.0/nullable-reference-types-specification.md",
                     "csharp-9.0/nullable-constructor-analysis.md",
                     "csharp-9.0/nullable-parameter-default-value-analysis.md",
@@ -621,7 +622,6 @@
                 "_csharplang/proposals/csharp-8.0/ranges.md": "Ranges and indices",
                 "_csharplang/proposals/csharp-8.0/using.md": "Pattern based using and using declarations",
                 "_csharplang/proposals/csharp-8.0/null-coalescing-assignment.md": "Null coalescing assignment",
-                "_csharplang/proposals/csharp-8.0/readonly-instance-members.md": "Readonly instance members",
                 "_csharplang/proposals/csharp-9.0/covariant-returns.md": "Covariant return types",
                 "_csharplang/proposals/csharp-9.0/extending-partial-methods.md": "Extending partial methods",
                 "_csharplang/proposals/csharp-9.0/extension-getenumerator.md": "Extension GetEnumerator support in foreach",
@@ -748,7 +748,6 @@
                 "_csharplang/proposals/csharp-8.0/ranges.md": "This feature specification describes the syntax for ranges and indices, which support indexing individual elements of a sequence or a range of a sequence from the start or end of that sequence.",
                 "_csharplang/proposals/csharp-8.0/using.md": "This feature specification supports pattern based using and using declarations to simplify resource cleanup.",
                 "_csharplang/proposals/csharp-8.0/null-coalescing-assignment.md": "This feature specification describes the syntax to support null coalescing assignment expressions using the '??=' operator.",
-                "_csharplang/proposals/csharp-8.0/readonly-instance-members.md": "This feature specification describes the syntax for declaring and using readonly instance members.",
                 "_csharplang/proposals/csharp-9.0/covariant-returns.md": "This feature specification describes covariant return types, where overriding member declarations can return a type derived from the overridden member declaration.",
                 "_csharplang/proposals/csharp-9.0/extending-partial-methods.md": "This feature specification describes extensions to partial methods. These extensions enable source generators to create or call partial methods.",
                 "_csharplang/proposals/csharp-9.0/extension-getenumerator.md": "This feature specification describes the necessary pattern for an extension method to provide the GetEnumerator support in a foreach loop.",

docs/csharp/language-reference/builtin-types/ref-struct.md

@@ -83,7 +83,7 @@ The compiler enforces these restrictions. If you write `ref struct` types that i
 For more information, see the following sections of the [C# language specification](~/_csharpstandard/standard/README.md):
 
 - [Structs: Ref modifier](~/_csharpstandard/standard/structs.md#1623-ref-modifier)
-- [Safe context constraint for ref struct types](~/_csharpstandard/standard/structs.md#16412-safe-context-constraint)
+- [Safe context constraint for ref struct types](~/_csharpstandard/standard/structs.md#16415-safe-context-constraint)
 
 For more information about `ref` fields, see the [Low-level struct improvements](~/_csharplang/proposals/csharp-11.0/low-level-struct-improvements.md) proposal note.
 

docs/csharp/programming-guide/generics/constraints-on-type-parameters.md

@@ -169,7 +169,7 @@ The preceding syntax would require implementers to use [explicit interface imple
 The `allows ref struct` anti-constraint declares that the corresponding type argument can be a [`ref struct`](../../language-reference/builtin-types/ref-struct.md) type. Instances of that type parameter must obey the following rules:
 
 - It can't be boxed.
-- It participates in [ref safety rules](~/_csharpstandard/standard/structs.md#16412-safe-context-constraint).
+- It participates in [ref safety rules](~/_csharpstandard/standard/structs.md#16415-safe-context-constraint).
 - Instances can't be used where a `ref struct` type isn't allowed, such as `static` fields.
 - Instances can be marked with the `scoped` modifier.
 

docs/csharp/specification/toc.yml

@@ -223,8 +223,6 @@ items:
       href: ../../../_csharplang/proposals/extensions.md
   - name: Structs
     items:
-    - name: Readonly instance members
-      href: ../../../_csharplang/proposals/csharp-8.0/readonly-instance-members.md
     - name: Parameterless struct constructors
       href: ../../../_csharplang/proposals/csharp-10.0/parameterless-struct-constructors.md
     - name: Auto-default struct