CI: Add Ruby 3.3 to build matrix, use checkout@v4 (#15)

.github/workflows/test.yaml

@@ -27,6 +27,7 @@ jobs:
           - "3.0"
           - "3.1"
           - "3.2"
+          - "3.3"
 
         experimental: [false]
 
@@ -46,7 +47,7 @@ jobs:
             ruby: "2.5"
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{matrix.ruby}}