chore: highlight variables are deprecated (#27987)

Issue number: N/A

---------

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## What is the current behavior?
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There was some confusion on
#27985 as to how the
`--highlight-*` variables on `ion-item` apply to the modern form
controls. These variables only work with the legacy form syntax, but we
did not add a proper deprecation message.

`--highlight-*` variables have been added to `ion-input` and
`ion-textarea` instead so developers can customize the highlight when
using the modern form syntax.

## What is the new behavior?
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this PR. -->

- Deprecated the `--highlight-*` variables on `ion-item`.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

Author: liamdebeasi

core/src/components/item/item.scss

@@ -2,6 +2,7 @@
 
 // Item
 // --------------------------------------------------
+// TODO: FW-4955
 
 :host {
   /**
@@ -45,10 +46,10 @@
    *
    * @prop --ripple-color: Color of the item ripple effect
    *
-   * @prop --highlight-height: The height of the highlight on the item
-   * @prop --highlight-color-focused: The color of the highlight on the item when focused
-   * @prop --highlight-color-valid: The color of the highlight on the item when valid
-   * @prop --highlight-color-invalid: The color of the highlight on the item when invalid
+   * @prop --highlight-height: The height of the highlight on the item. Only applies to inputs and textareas using the legacy form syntax. DEPRECATED: Highlights can be styled on `ion-input` or `ion-textarea` when using the modern form syntax.
+   * @prop --highlight-color-focused: The color of the highlight on the item when focused. Only applies to inputs and textareas using the legacy form syntax. DEPRECATED: Highlights can be styled on `ion-input` or `ion-textarea` when using the modern form syntax.
+   * @prop --highlight-color-valid: The color of the highlight on the item when valid. Only applies to inputs and textareas using the legacy form syntax. DEPRECATED: Highlights can be styled on `ion-input` or `ion-textarea` when using the modern form syntax.
+   * @prop --highlight-color-invalid: The color of the highlight on the item when invalid. Only applies to inputs and textareas using the legacy form syntax. DEPRECATED: Highlights can be styled on `ion-input` or `ion-textarea` when using the modern form syntax.
    */
   --border-radius: 0px;
   --border-width: 0px;