ci: update actions/checkout to v4 (#4)

the other versions are no longer supported

Author: mattr-

.github/workflows/gem.yml

@@ -19,7 +19,7 @@ jobs:
           - 2.7
     steps:
       - name: Checkout Repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: "Set up Ruby ${{ matrix.ruby_version }}"
         uses: ruby/setup-ruby@v1
         with: