Editor: Bump pinned hash for the Gutenberg repository.

This updates the pinned hash from the `gutenberg` from `c15cef1d6b07f666df28dac0383bafb0edfe0914 ` to `3a4e8d1418d25da83b70158bcaabf65580690b6b`.

The following changes are included:

- [WP.7.0] Admin UI: Backport accessibility fixes (https://github.com/WordPress/gutenberg/pull/77617, https://github.com/WordPress/gutenberg/pull/78001) (https://github.com/WordPress/gutenberg/pull/78002)
- Fix: Shortcode block does not render in Navigation Overlay (https://github.com/WordPress/gutenberg/pull/77511)
- feat: Enhance Connectors page on read-only file system (https://github.com/WordPress/gutenberg/pull/77521)
- Connectors: Avoid using centered text (https://github.com/WordPress/gutenberg/pull/78125)
- Revisions: Add tooltip to diff marker buttons  (https://github.com/WordPress/gutenberg/pull/77690)
- Add backport for WP_ALLOW_COLLABORATION (https://github.com/WordPress/gutenberg/pull/78160)
- Add aria-label to Revisions button in Post Summary sidebar (https://github.com/WordPress/gutenberg/pull/78140)
- Revisions diff markers: enforce 24×24px minimum target size (WCAG 2.5.8) (https://github.com/WordPress/gutenberg/pull/77671)
- Connectors: Replace @wordpress/ui Link and Notice usage (https://github.com/WordPress/gutenberg/pull/78117)
- Connectors: Increase right padding of callout for mobile layout (https://github.com/WordPress/gutenberg/pull/78126)
- isFulfilled: don't change resolution state, call in resolveSelect (https://github.com/WordPress/gutenberg/pull/78201)
- Connectors: Restyle AI plugin callout with pastel background and beaker decoration (https://github.com/WordPress/gutenberg/pull/78243)
- Block supports: Optimize custom CSS class rendering and parsing (https://github.com/WordPress/gutenberg/pull/78217)
- Block Inspector: Hide Styles tab in preview mode (https://github.com/WordPress/gutenberg/pull/78230)
- Navigation Link: Preserve custom labels during link updates (https://github.com/WordPress/gutenberg/pull/77186)
- Editor: Fix Visual Revisions meta keys overlap (https://github.com/WordPress/gutenberg/pull/78156)
- Editor: Disable Visual Revisions when classic meta boxes are present (https://github.com/WordPress/gutenberg/pull/78249) (https://github.com/WordPress/gutenberg/pull/78286)
- Revisions: Scale diff markers width with user text-size preference (https://github.com/WordPress/gutenberg/pull/78273)

A full list of changes can be found on GitHub: https://github.com/WordPress/gutenberg/compare/c15cef1d6b07f666df28dac0383bafb0edfe0914…3a4e8d1418d25da83b70158bcaabf65580690b6b.

Log created with:

`git log --reverse --format="- %s" c15cef1d6b07f666df28dac0383bafb0edfe0914..3a4e8d1418d25da83b70158bcaabf65580690b6b | sed 's|#\([0-9][0-9]*\)|https://github.com/WordPress/gutenberg/pull/\1|g; /github\.com\/WordPress\/gutenberg\/pull/!d' | pbcopy`

Reviewed by desrosj.
Merges [62360] to `trunk`.

See #64595.

git-svn-id: https://develop.svn.wordpress.org/trunk@62361 602fd350-edb4-49c9-b593-d223f7449a82