chore: update librarian to v0.21.0

[zhumin8]

Weekly update to librarian version and re-generate. This update includes a generator upgrade, see https://github.com/googleapis/librarian/releases/tag/v0.21.0.

Manually changes applied changes to postprocess config files due to generation failure after upgrade. 46dfa75

commands run:

go run github.com/googleapis/librarian/cmd/librarian@latest update version
V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
go run github.com/googleapis/librarian/cmd/librarian@${V} tidy
# Build a new Docker image
go run github.com/googleapis/librarian/tool/cmd/builddockerimages@latest --language python --version=${V}
# Regenerate all libraries in Docker. Around 2 minutes.
docker run -u $(id -u):$(id -g) -v .:/repo -v ~/.cache:/.cache -w /repo docker.io/library/librarian-python:${V}  generate -v --all

For the sake of easier review, separated 3 commits:

• librarian version update f60299a
• Manual changes to config to make fix generation error: 46dfa75
• Generated code changes: b8f2240

Fixes googleapis/librarian#6450

[gemini-code-assist]

Code Review

This pull request updates dependency versions across multiple packages, bumping google-api-core to 2.24.2, protobuf to 6.33.5, proto-plus to 1.26.1, and grpc-google-iam-v1 to 0.14.2, along with updating setup.py version parsing regexes. A critical issue was identified in bigtable-integration.yaml where the paths block was accidentally deleted, breaking the YAML structure.