Use latest Ubuntu images

Jobs in those workflows has little to do with specific OS version, so
just use the latest.

.github/workflows/file_health.yaml

@@ -5,7 +5,7 @@ on: [pull_request, push]
 jobs:
   check_file_health:
     name: Check file health
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: checkout repository
         uses: actions/checkout@v2

.github/workflows/webui_ci.yaml

@@ -5,7 +5,7 @@ on: [pull_request, push]
 jobs:
   check_webui:
     name: Check WebUI
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     defaults:
       run:
         working-directory: src/webui/www