gha: switch to ubuntu-latest for php-cs-fixer

Author: mfn

.github/workflows/analysis.yml

@@ -51,7 +51,7 @@ jobs:
 
   code-style-checker:
     name: Code Style checker
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v2