Do not watch v5.18.x

This change removes the watch for v5.18.x releases since it is marked
EOL.

.github/workflows/watch.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: [ubuntu-latest]
     strategy:
       matrix:
-        branch: ['v5.19.x', 'v5.18.x']
+        branch: ['v5.19.x']
     steps:
     - name: Checkout Sources
       uses: actions/checkout@v2