Use actions/checkout@v4

v2 uses node 12, which has been deprecated for a while

.github/workflows/ci.yml

@@ -58,7 +58,7 @@ jobs:
         options: --health-cmd="mysqladmin ping" --health-interval=10s --health-timeout=5s --health-retries=3
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: ./.github/actions/test
         with:
           ruby-version: ${{ matrix.ruby }}