Elements: Fix block instance element styles for links applying to buttons

[aaronrobertshaw]

This ticket tracks the backporting of PHP files for the following Gutenberg changes:

​https://github.com/WordPress/gutenberg/pull/59114

The previous selector used for link styles set on an individual block instance simply used the a element. This meant it incorrectly was applied to buttons as well.

The linked Gutenberg PR fixes this by updating the selector to match the one used in theme.json i.e. a:where(:not(.wp-element-button)).

Syncs the changes from ​https://github.com/WordPress/gutenberg/pull/59114

These changes fix the selector used by block instance element styles for links so they do not get incorrectly applied to buttons.

To test:

1. Run: npm run test:php -- --filter Tests_Block_Supports_WpRenderElementsSupport
2. Edit a post, add a group block containing a paragraph with a link and a button
3. Select the group block and apply a color to links within that block
4. Save and view the post on the frontend, the button's color should match the editor and not be impacted by the link color previously selected

Trac ticket: https://core.trac.wordpress.org/ticket/60557

[youknowriad]

In 57664:

Editor: Fix instance element styles for links applying to buttons.

These changes fix the selector used by block instance element styles for links so they do not get incorrectly applied to buttons.

Props aaronrobertshaw, freewebmentor, mukesh27.
Fixes #60557.

Commit https://core.trac.wordpress.org/changeset/57664