Base Styles: Update z-index for snackbars to ensure proper layering with modals

[yogeshbhutkar]

What, Why & How?

closes: #68728

This PR addresses the bug where the Snackbar gets displayed below the Dialog by incrementing the z-index.

Testing Instructions

1. Navigate to the post edit screen.
2. Open the Block Inserter.
3. Navigate to the Patterns tab and open the Explore all patterns dialog.
4. Try selecting a Pattern from the Dialog.
5. Confirm that the snackbar now gets rendered above the dialog.

Screencast

PR.mov

[github-actions]

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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[Mamaduka]

A small unrelated suggestion, @yogeshbhutkar! You might want to place an issue reference just under the first heading; this makes it easier to spot the associated issue.

[yogeshbhutkar]

Thanks for the suggestion @Mamaduka. Updated the PR description.