github: add Ruby 3.4 for CI

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

.github/workflows/linux.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.3', '3.2', '3.1', '3.0' ]
+        ruby: [ '3.4', '3.3', '3.2', '3.1', '3.0' ]
         os:
           - ubuntu-latest
         rdkafka_versions: