Fix: Button highlight popover overflow conflict with link popover

[delowardev]

Fixes: #38339

Rendering button link popover in the global slot causing z-index conflict/overlapping issue with text highlight popover; Using block-toolbar slot solves the overlapping issue here.

Description

Testing Instructions

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Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[ndiego]

Just tested and works as expected, thanks for the PR @delowardev!

@Mamaduka and @ntsekouras, whenever you have a second, can I get a sanity check on this? Just want to make sure the use of __unstableSlotName is the direction we want to go here.

[delowardev]

(Just for the context) This bug appeared after #34956. Here the block tools popover slot had changed, but the button link popover was left in the previous slot position which created z-index conflict.

[ntsekouras]

Thanks for the PR and the context about the issue @delowardev!

[ellatrix]

Makes sense to me!