Build/Test Tools: Pin a Gutenberg version in the 6.6 branch.

The minimum required version of WordPress for the Gutenberg plugin will be bumped to `6.7` in version `20.8.0`. This pins the final version with support for WordPress 6.6 to ensure E2E tests continue to pass.

Props mamaduka.
Fixes #63377.

git-svn-id: https://develop.svn.wordpress.org/branches/6.6@60214 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/workflows/end-to-end-tests.yml

@@ -48,6 +48,7 @@ jobs:
         LOCAL_SCRIPT_DEBUG: [ true, false ]
     with:
       LOCAL_SCRIPT_DEBUG: ${{ matrix.LOCAL_SCRIPT_DEBUG }}
+      gutenberg-version: '20.7.0'
 
   slack-notifications:
     name: Slack Notifications