Upgrade actions/checkout

Author: davishmcclurg

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
             ruby: truffleruby-head
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}