fix: remove animation fill mode from modal frame

[miguelpeixe]

Description

Fixes the positioning of Popovers inside Modals for Chrome and Safari.

With the animation fill mode set to forwards, the final step sets transform to a defined value. This makes position: fixed child elements position from the Modal's edge instead of the viewport.

More on position and containing blocks: https://developer.mozilla.org/en-US/docs/Web/CSS/Containing_block#identifying_the_containing_block

Closes #8468.

Testing Instructions

1. Using Chrome or Safari (this bug does not occur on Firefox), create a Modal with child Popover. Observe that it's mispositioned.
2. Check out this branch and observe the Popover is now properly positioned.

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.

[github-actions]

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[ndiego]

@miguelpeixe I just tested #8468 and it appears to no longer be an issue in WP 6.0+. Therefore, while this PR is greatly appreciated, it likely is no longer needed. Can you confirm on your end that the positioning of popovers in modals is working correctly now?

[miguelpeixe]

Thank you for reaching back, @ndiego! I was also able to confirm that this is no longer an issue. Closing the PR.