CI: Bump actions/checkout to v4

The checkout v2, the version currently in use on CI, depends on node12
which is going to be removed soon. Let's use the latest version in order
to get rid of deprecation warnings in logs and remain compatible with
GitHun runners.

Author: ikalnytskyi

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
           - windows-latest
           - macos-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Run setup-postgres
         uses: ./
@@ -44,7 +44,7 @@ jobs:
           - windows-latest
           - macos-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Run setup-postgres
         uses: ./