chore(librarian): update gapic-generator to 1.30.6 (#15500)

This PR is needed to address the issue in
googleapis/gapic-generator-python#2548.

See googleapis/gapic-generator-python#2549 for
more details. The release notes and changes for generated client
libraries will be added in a separate PR.

Author: parthea

.generator/requirements.in

@@ -1,5 +1,5 @@
 click
-gapic-generator==1.30.5 # Fix mypy issue https://github.com/googleapis/gapic-generator-python/pull/2536
+gapic-generator==1.30.6 # Fix incorrect REST serialization https://github.com/googleapis/gapic-generator-python/pull/2549
 nox
 starlark-pyo3>=2025.1
 build