Bump cache version

Author: malberts

.github/workflows/ci.yml

@@ -44,7 +44,7 @@ jobs:
             mediawiki
             !mediawiki/extensions/
             !mediawiki/vendor/
-          key: mw_${{ matrix.mw }}-php${{ matrix.php }}_v3
+          key: mw_${{ matrix.mw }}-php${{ matrix.php }}_v4
 
       - name: Cache Composer cache
         uses: actions/cache@v2