Maintain actions.

.github/workflows/codeql.yaml

@@ -2,10 +2,7 @@ name: CodeQL
 
 "on":
   workflow_dispatch: {}
-  push:
-    branches: [main]
-  pull_request:
-    branches: [main]
+  pull_request: {}
   schedule:
     - cron: '0 0 * * *'
 
@@ -17,7 +14,7 @@ jobs:
       actions: read
       contents: read
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v2.4.0
       - uses: github/codeql-action/init@v1
         with:
           languages: java