Specify specific ubuntu version

To try to work around:

```
Error: The current runner (ubuntu-24.04-x64) was detected as self-hosted
because the platform does not match a GitHub-hosted runner image (or
that image is deprecated and no longer supported).
```

Author: ramhoj

.github/workflows/ruby.yml

@@ -19,10 +19,10 @@ permissions:
 jobs:
   test:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.4']
+        ruby-version: ['2.6', '2.7', '3.0', '3.1.4']
 
     steps:
     - uses: actions/checkout@v4