ci: Pin test runner to ubuntu-24.04 for Ruby 3.0 support (#115)

Author: khvn26

.github/workflows/pull_request.yaml

@@ -25,7 +25,8 @@ permissions:
 jobs:
   test:
     name: CI
-    runs-on: ubuntu-latest
+    # Ruby 3.0 has no prebuilt binaries for ubuntu-26.04 (now ubuntu-latest).
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
         ruby-version: ['3.0', '3.1', '3.2', '3.3']