CI: Remove Ruby 2.4 and 2.5 from test matrix

Those are old versions, and apparently fail on CI now.
https://github.com/crate/crate_ruby/actions/runs/21842711977/job/63030941887

Author: amotl

.github/workflows/tests.yml

@@ -24,7 +24,14 @@ jobs:
       fail-fast: false
       matrix:
         os: [ "ubuntu-22.04", "macos-15-intel" ] # , windows-latest ]
-        ruby-version: [ "2.4", "2.5", "2.6", "2.7", "3.0", "3.1", "3.2", "3.3" ]
+        ruby-version: [
+          "2.6",
+          "2.7",
+          "3.0",
+          "3.1",
+          "3.2",
+          "3.3",
+        ]
 
     name: Ruby ${{ matrix.ruby-version }} on OS ${{ matrix.os }}
     steps: