Pin runner image to specific version to decouple from GitHub updates

Signed-off-by: Martin Schurz <[email protected]>

.github/workflows/prettier-md.yml

@@ -11,7 +11,7 @@ on:  # yamllint disable-line rule:truthy
 jobs:
   prettier-md:
     if: github.repository == 'dev-sec/ansible-collection-hardening'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     timeout-minutes: 1
 
     steps: