Merge branch '473624-deprecate-update-namespace-endpoint' into 'master'

Add [DEPRECATED] label to update namespace

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/176660



Merged-by: Marcos Rocha <mrocha@gitlab.com>
Approved-by: Dominic Bauer <dbauer@gitlab.com>
Approved-by: Marcos Rocha <mrocha@gitlab.com>
Co-authored-by: Bishwa Hang Rai <bhrai@gitlab.com>