chore(librarian): bump gapic-generator to 1.28.1 (#14717)

See related issue : googleapis/librarian#2486

1.28.1 is needed to resolve an issue with `gapic-version` option not
working for `google-cloud-biglake`. An issue was fixed in
`gapic-generator==1.28.1`
https://github.com/googleapis/gapic-generator-python/releases/tag/v1.28.1
via googleapis/gapic-generator-python#2460

Author: parthea

.generator/requirements.in

@@ -1,5 +1,5 @@
 click
-gapic-generator>=1.28.0 # Python 3.14 support
+gapic-generator>=1.28.1 # Python 3.14 support, gapic-version support
 nox
 starlark-pyo3>=2025.1
 build