Add ability to lock discussions in work items

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/141281



Merged-by: Tomas Bulva <tbulva@gitlab.com>
Approved-by: Nick Leonard <nleonard@gitlab.com>
Approved-by: Ammar Alakkad <aalakkad@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Tomas Bulva <tbulva@gitlab.com>
Reviewed-by: Ammar Alakkad <aalakkad@gitlab.com>
Reviewed-by: Coung Ngo <cngo@gitlab.com>
Reviewed-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Coung Ngo <cngo@gitlab.com>