Remove release branch staleness tests.

These are now covered by our daily testing of release branches

.github/scripts/validate_yaml.py

@@ -65,35 +65,3 @@
               % (step['name'], job)
           )
 print('PASSED: All steps in all jobs check the continuous-run condition.')
-
-# Check to make sure the list of included branches matches the list of excluded
-# branches in staleness_check.yml.
-with open(
-    os.path.join(os.path.dirname(__file__), '../workflows/staleness_check.yml'),
-    'r',
-) as f:
-  regex_pattern = r"'(\d+\.x)'"
-  data = yaml.safe_load(f)
-  matrix = data['jobs']['test']['strategy']['matrix']
-  included_branches = matrix['branch']
-  # Main should be included in all test runs
-  included_branches.remove('main')
-  excludes = matrix['exclude']
-  for entry in excludes:
-    match = re.search(regex_pattern, entry['branch'])
-    branch = match.group(1)
-    if branch not in included_branches:
-      raise ValueError(
-          'Branch %s is excluded for presubmit runs but is not in the list of'
-          ' matrix branches in staleness_check.yml.' % branch
-      )
-    included_branches.remove(branch)
-  if included_branches:
-    raise ValueError(
-        'Branches %s are in the list of matrix branches but do not get excluded'
-        ' for presubmit runs in staleness_check.yml.' % included_branches
-    )
-  print(
-      'PASSED: The list of included branches matches the list of excluded'
-      ' branches in staleness_check.yml.'
-  )

.github/workflows/staleness_check.yml

@@ -20,26 +20,14 @@ on:
 permissions: {}
 jobs:
   test:
-    strategy:
-      fail-fast: false
-      matrix:
-        # TODO Remove this parameterization once the release branch testing is stable.
-        branch: [main, 25.x, 27.x, 28.x]
-        os: [{ name: Linux, value: ubuntu-latest}]
-        exclude:
-          # If we are in a presubmit run, only test main
-          - branch: ${{ !inputs.continuous-run && '25.x' }}
-          - branch: ${{ !inputs.continuous-run && '27.x' }}
-          - branch: ${{ !inputs.continuous-run && '28.x' }}
-
-    name: Test staleness ${{ matrix.os.name }} ${{ github.head_ref && 'PR' || matrix.branch }}
-    runs-on: ${{ matrix.os.value }}
+    name: Test staleness
+    runs-on: ubuntu-latest
     if: ${{ github.event.repository.full_name == 'protocolbuffers/protobuf' }}
     steps:
-      - name: Checkout ${{ github.head_ref && 'PR' || matrix.branch }}
+      - name: Checkout
         uses: protocolbuffers/protobuf-ci/checkout@v3
         with:
-          ref: ${{ inputs.safe-checkout || github.head_ref || matrix.branch }}
+          ref: ${{ inputs.safe-checkout || github.head_ref || github.ref }}
 
       - name: Mark runs associated with commits
         if: ${{ github.event_name != 'schedule' && github.event_name != 'workflow_dispatch' }}