gopls/doc/release: update gopls release notes for language support

Update the v0.16.0 release notes to reflect 1.23 support and changes to
our support policy.

For golang/go#67936

Change-Id: I330b8a3897fc3b4b944eb695143042654556dbf1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/592156
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

gopls/README.md

@@ -85,8 +85,8 @@ gopls build will depend on the latest version of Go. However, due to the
 [forward compatibility](https://go.dev/blog/toolchain) support added to the
 `go` command in Go 1.21, as long as Go 1.21 or later are used to install gopls,
 the toolchain upgrade will be handled automatically, just like any other
-dependency. Gopls will continue to support integrating with the two most recent
-major Go releases of the `go` command, per the Go Release Policy. See
+dependency. Gopls will continue to support integrating with the three most
+recent major Go releases of the `go` command. See
 [issue #65917](https://go.dev/issue/65917) for more details.
 
 Maintaining support for legacy versions of Go caused

gopls/doc/release/v0.16.0.md

@@ -1,9 +1,35 @@
-gopls/v0.16.0
+# gopls/v0.16.0
 
+<!-- TODO: update this instruction once v0.16.0 is released -->
 ```
-go install golang.org/x/tools/[email protected]
+go install golang.org/x/tools/[email protected]-pre.1
 ```
 
+This release includes several features and bug fixes, and is the first
+version of gopls to support Go 1.23. To install it, run:
+
+## New support policy; end of support for Go 1.19 and Go 1.20
+
+This is the last release of gopls that may be built and used with Go 1.19 or Go
+1.20. If built or used with either of these Go versions, it will display
+a message advising the user to upgrade.
+
+Starting with [email protected], which will be released after Go 1.23.0 is released
+in August, installing gopls will require the latest version of the Go
+toolchain. Thanks to the [forward compatibility](https://go.dev/blog/toolchain)
+added to Go 1.21, any necessary toolchain upgrade should be handled
+automatically for users of Go 1.21 or later. This restriction is for _building_
+gopls only -- we will continue to support command line integration with the
+last three major Go versions. Nonetheless, this is fewer than our previous policy
+of gopls supporting the last four major Go versions of installed toolchains.
+
+See the newly updated
+[support policy](https://github.com/golang/tools/tree/master/gopls#support-policy)
+for details. We expect that this change will significantly reduce our
+maintenance burden, and will help improve the stability of future gopls
+releases. Please comment on [issue #65917](https://go.dev/issue/65917) if you
+have concerns about this change.
+
 ## Configuration Changes
 
 - The experimental "allowImplicitNetworkAccess" setting is deprecated (but not
@@ -12,6 +38,15 @@ go install golang.org/x/tools/[email protected]
 
 ## New features
 
+### Go 1.23 support
+
+This version of gopls is the first to support new Go 1.23 language features,
+including
+[range-over-func](https://go.dev/wiki/RangefuncExperiment) iterators
+and support for the
+[`godebug` directive](https://go.dev/ref/mod#go-mod-file-godebug)
+in go.mod files.
+
 ### Integrated documentation viewer
 
 Gopls now offers a "View package documentation" code action that opens