Build/Test Tools: Add environment variable for current release.

This adds a global environment variable to the Test old branches workflow to make the supported version more clear and easier to update in the future. This will also make it easier to reference in more places as this workflow grows.

See #58867.

git-svn-id: https://develop.svn.wordpress.org/trunk@56955 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -16,6 +16,9 @@ on:
 # Any needed permissions should be configured at the job level.
 permissions: {}
 
+env:
+  CURRENTLY_SUPPORTED_BRANCH: '6.3'
+
 jobs:
   dispatch-workflows-for-old-branches:
     name: ${{ matrix.workflow }} for ${{ matrix.branch }}
@@ -85,7 +88,7 @@ jobs:
     steps:
       - name: Dispatch workflow run
         uses: actions/github-script@d7906e4ad0b1822421a7e6a35d5ca353c962f410 # v6.4.1
-        if: ${{ github.event_name == 'push' || github.event.schedule == '0 0 15 * *' || matrix.branch == '6.3' }}
+        if: ${{ github.event_name == 'push' || github.event.schedule == '0 0 15 * *' || matrix.branch == env.CURRENTLY_SUPPORTED_BRANCH }}
         with:
           retries: 2
           retry-exempt-status-codes: 418