Editor: Bump pinned hash for the Gutenberg repository.

This updates the pinned hash from the `gutenberg` from `9b8144036fa5faf75de43d4502ff9809fcf689ad` to `8c78d87453509661a9f28f978ba2c242d515563b`.

The following changes are included:

- Navigation Editor: Allow any blocks to be inserted by gating contentOnly insertion rules to section blocks (https://github.com/WordPress/gutenberg/pull/76189)
- Add `fetchpriority=low` to `IMG` tags in collapsed Details blocks (https://github.com/WordPress/gutenberg/pull/76269)
- Connectors: Add logo URL support for custom AI providers (https://github.com/WordPress/gutenberg/pull/76190)
- Cover Block: Add a playlist parameter to loop YouTube background videos. (https://github.com/WordPress/gutenberg/pull/76004)
- Connectors: Memoize getConnectors selector (https://github.com/WordPress/gutenberg/pull/76339)
- HTML Block: Fix broken layout (https://github.com/WordPress/gutenberg/pull/76278)
- Tests: Skip connector logo URL tests when AI Client is unavailable (https://github.com/WordPress/gutenberg/pull/76343)
- Navigation Overlay: Explicitly set fetchpriority for images (https://github.com/WordPress/gutenberg/pull/76208)
- Connectors: Show API key source for env vars and wp-config constants (https://github.com/WordPress/gutenberg/pull/76355)
- Connectors: Move API key validation and masking to REST dispatch level (https://github.com/WordPress/gutenberg/pull/76327)
- Connectors: Replace apiFetch with core-data store selectors (https://github.com/WordPress/gutenberg/pull/76333)
- Do not sync local attributes (https://github.com/WordPress/gutenberg/pull/76267)
- Add `fetchpriority=low` to `IMG` tags in collapsed Accordion Item blocks (https://github.com/WordPress/gutenberg/pull/76336)
- Implement disconnection debounce after initial connection (https://github.com/WordPress/gutenberg/pull/76114)
- Allow Post Content to be edited when 'Show template' is active and Post content is nested in a Template Part (https://github.com/WordPress/gutenberg/pull/76305)
- Fix: Document Bar: Back button flickers (https://github.com/WordPress/gutenberg/pull/76320)
- RTC: Move event hooks from editor to core-data (https://github.com/WordPress/gutenberg/pull/76358)
- fix(navigation): prevent right-justified submenu overflow in custom overlays (https://github.com/WordPress/gutenberg/pull/76360)
- Connectors: Add connectors registry for extensibility (https://github.com/WordPress/gutenberg/pull/76364)
- Connectors: Add empty state when no connectors are registered (https://github.com/WordPress/gutenberg/pull/76375)
- Temp: Disable RTC in the site editor (https://github.com/WordPress/gutenberg/pull/76223)
- Connectors: Add AI Experiments plugin callout with install/activate functionality (https://github.com/WordPress/gutenberg/pull/76379)
- Editor: Polish real-time collaboration presence UI and move Avatar to editor package (https://github.com/WordPress/gutenberg/pull/75652) (https://github.com/WordPress/gutenberg/pull/76365)
- RTC: Add collaborator selection highlighting in rich text (https://github.com/WordPress/gutenberg/pull/76107)
- Sync changes from `wp_enqueue_global_styles()` to Gutenberg override (https://github.com/WordPress/gutenberg/pull/76127)
- [RTC] Fix performance regression on post save (https://github.com/WordPress/gutenberg/pull/76370)
- Media: Enable AVIF support for client-side uploads (https://github.com/WordPress/gutenberg/pull/76371)
- Connectors: Move plugin status computation to script module data (https://github.com/WordPress/gutenberg/pull/76409)
- Revisions: Skip rendered fields in REST API responses (https://github.com/WordPress/gutenberg/pull/76347)
- E2E Tests: Add connector setup flow tests with test AI provider (https://github.com/WordPress/gutenberg/pull/76433)
- RTC: Place sync connection modal in front of popover (https://github.com/WordPress/gutenberg/pull/76431)
- Connectors: Sync PHP code with WordPress Core (https://github.com/WordPress/gutenberg/pull/76443)
- Editor: Show own presence in collaborative editing sessions (https://github.com/WordPress/gutenberg/pull/76413) (https://github.com/WordPress/gutenberg/pull/76445)

A full list of changes can be found on GitHub: https://github.com/WordPress/gutenberg/compare/9b8144036fa5faf75de43d4502ff9809fcf689ad…8c78d87453509661a9f28f978ba2c242d515563b.

Log created with:

git log --reverse --format="- %s" 9b8144036fa5faf75de43d4502ff9809fcf689ad..8c78d87453509661a9f28f978ba2c242d515563b | sed 's|#\([0-9][0-9]*\)|https://github.com/WordPress/gutenberg/pull/\1|g; /github\.com\/WordPress\/gutenberg\/pull/!d' | pbcopy

See #64595.


git-svn-id: https://develop.svn.wordpress.org/trunk@61988 602fd350-edb4-49c9-b593-d223f7449a82