CI: Run builds when targeting stable branches

Allow running the "build" job when targeting stable branches. If it was
possible to use a regular expression to match branch names, this would
use

  ^wpewebkit-[0-9]+\.[0-9]+$

but as that is not possible, this matches instead branches with the
"wpewebkit-2." prefix followed by two more characters (matching only
numbers is not possible).

Author: aperezdc

.github/workflows/build.yml

@@ -20,6 +20,7 @@ on:
       - closed
     branches:
       - 'main'
+      - 'wpewebkit-2.??'
 
 jobs:
   build: