CI: bump actions/checkout from 2 to 3

Resolves the GitHub Actions deprecation:

Node.js 12 actions are deprecated. Please update the following actions to use
Node.js 16: actions/checkout@v2. For more information see:
https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

.github/workflows/ruby.yml

@@ -23,7 +23,7 @@ jobs:
         ruby-version: [2.5, 2.6, 2.7, '3.0', 3.1, 3.2, head, debug, truffleruby]
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') || matrix.ruby == 'debug' }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install libcurl header
       run: |
         if ${{ matrix.os == 'macos' }}