Build/Test Tools: Update the currently supported branch that's used to determine the frequency of scheduled tests of old branches.

See #63170


git-svn-id: https://develop.svn.wordpress.org/trunk@60323 602fd350-edb4-49c9-b593-d223f7449a82

Author: johnbillion

.github/workflows/test-old-branches.yml

@@ -25,7 +25,7 @@ on:
 permissions: {}
 
 env:
-  CURRENTLY_SUPPORTED_BRANCH: '6.7'
+  CURRENTLY_SUPPORTED_BRANCH: '6.8'
 
 jobs:
   dispatch-workflows-for-old-branches: