Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/testing.yml

@@ -45,13 +45,13 @@ jobs:
 
     - name: Check out code base
       if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
-      uses: actions/checkout@8f4b7f84864484a7bf31766abe9204da3cbe65b3 # v3.5.0
+      uses: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871 # v4.2.1
       with:
         fetch-depth: 0
 
     - name: Check out code base
       if: github.event_name == 'pull_request'
-      uses: actions/checkout@8f4b7f84864484a7bf31766abe9204da3cbe65b3 # v3.5.0
+      uses: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871 # v4.2.1
       with:
         fetch-depth: 0
         ref: ${{ github.event.pull_request.head.sha }}