pin ubuntu version to 22.04 (try to fix https://github.com/Kozea/Radicale/issues/1594)

.github/workflows/generate-documentation.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   generate:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
         with: