Update list of Ruby versions to test against

Removes all of the versions which have reached end of life. Ruby 3.4
won't be released for another six days and I'm not sure if it will even
work despite rc1 being out.

.github/workflows/tests.yml

@@ -13,14 +13,14 @@ jobs:
     strategy:
       matrix:
         ruby_version:
-          - "2.6"
-          - "2.7"
-          - "3.0"
           - "3.1"
+          - "3.2"
+          - "3.3"
+          - "3.4"
           - "head"
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1