github: update target version of Ruby 3.4

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

Author: kenhys

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        ruby: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4.0-preview2']
+        ruby: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4']
         include:
           - os: macos-13
             ruby: '2.5'