chore: Add library_generation.Dockerfile to release please config. (#…

…3179)

The [generator
version](https://github.com/googleapis/sdk-platform-java/blob/4f2f9e027d1413e1beca1092c20e2e542227ca28/.cloudbuild/library_generation/library_generation.Dockerfile#L21-L23)
in Dockerfile is not updated by release please, because it is not added
to release please config. See
[errors](https://github.com/googleapis/sdk-platform-java/actions/runs/10776097913/job/29883280148?pr=3139#logs)
in release PR for details.

release-please-config.json

@@ -11,8 +11,8 @@
         ".cloudbuild/graalvm/cloudbuild-test-a.yaml",
         ".cloudbuild/graalvm/cloudbuild-test-b.yaml",
         ".cloudbuild/library_generation/cloudbuild-library-generation-push.yaml",
-        "generation_config.yaml"
+        ".cloudbuild/library_generation/library_generation.Dockerfile"
       ]
     }
   }
-}
+}