fix: replace deprecated EditorSnackbars with SnackbarNotices

[dcalhoun]

What?

Replace the deprecated wp.editor.EditorSnackbars component with wp.notices.SnackbarNotices.

Why?

wp.editor.EditorSnackbars was deprecated in @wordpress/editor v7.0 and will be removed in v7.2, generating a deprecation warning in the console on every editor load:

[Warning] wp.editor.EditorSnackbars is deprecated since version 7.0 and will be removed in version 7.2. Please use wp.notices.SnackbarNotices instead.

How?

• Swap EditorSnackbars (from @wordpress/editor) for SnackbarNotices (from @wordpress/notices) in layout/index.jsx
• Replace the internal WordPress class components-editor-notices__snackbar with a project-owned gutenberg-kit-layout__snackbar class
• Move the snackbar positioning styles from editor/style.scss into layout/style.scss, co-locating them with the component that renders the snackbar

Testing Instructions

1. Open the editor in your desktop web browser by running make dev-server and navigating to the local URL with the ?dev_mode parameter appended (e.g. http://localhost:5173/?dev_mode).
2. Open the browser console (Web Inspector).
3. Verify the following deprecation warning is no longer present:

   wp.editor.EditorSnackbars is deprecated since version 7.0 and will be removed in version 7.2. Please use wp.notices.SnackbarNotices instead.

4. Trigger a snackbar notice (e.g. copy all content via the editor menu) and verify it displays correctly.

Accessibility Testing Instructions

N/A — no user-facing UI changes.

Screenshots or screencast

N/A — no user-facing UI changes.

[adalpari]

Works as expected! 👏 👏