ci: upgrade actions/checkout to v3

Author: G-Rath

.github/workflows/rubocop.yml

@@ -12,7 +12,7 @@ jobs:
         ruby: ['2.7', '3.0']
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}