CI: use v3 of GitHub Actions checkout

Author: olleolleolle

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     name: Rubocop
     runs-on: 'ubuntu-20.04'
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: '3.0'
@@ -73,7 +73,7 @@ jobs:
 
     env: ${{ matrix.env }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}