Update actions/checkout action to v3

Author: renovate[bot]

.github/workflows/pull_request.yml

@@ -10,7 +10,7 @@ jobs:
         ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0' ]
     name: Ruby v${{ matrix.ruby }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}