update ubuntu version (#271)

Author: martin0995

.github/workflows/check-mkdocs-build.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   build-docs-site:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v5