CI: Update actions/checkout

Author: ledermann

.github/workflows/main.yml

@@ -38,7 +38,7 @@ jobs:
       BUNDLE_GEMFILE: gemfiles/Gemfile-activemodel-${{ matrix.rails }}.x
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
     - name: Setup Ruby
       uses: ruby/setup-ruby@v1