chore: update librarian to v0.21.1-0.20260617000028-820646f3db93

[zhumin8]

Weekly update of librarian version and re-generate. Use pseudo version because v0.21.0 has a known issue for Java.
Command used:

go run github.com/googleapis/librarian/cmd/librarian@latest update version

# Get the updated version
V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)

# Regenerate everything to pick up librarian / sdk.yaml changes.
go run github.com/googleapis/librarian/cmd/librarian@${V} generate --all

[gemini-code-assist]

Code Review

This pull request updates the version of the Java language configuration in librarian.yaml from v0.20.0 to v0.21.1-0.20260617000028-820646f3db93. There are no review comments, and I have no feedback to provide.