CI: Upgrade actions/checkout to v3

.github/workflows/gramps-ci.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-20.04
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install dependencies
       run: |
         sudo apt-get update