CI: Use actions/checkout@v4, add Ruby 3.3

.github/workflows/ci.yml

@@ -14,11 +14,12 @@ jobs:
     strategy:
       matrix:
         ruby-version:
+          - "3.3"
           - "3.2"
           - "3.1"
           - "3.0"
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby-version }}
         uses: ruby/setup-ruby@v1
         with:
@@ -32,7 +33,7 @@ jobs:
     environment: Rubygems
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: