Merge branch '429853-remove-remove_mr_blocking_constraints-feature-flag' into 'master'

Remove remove_mr_blocking_constraints feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/136775



Merged-by: Bob Van Landuyt <bob@gitlab.com>
Approved-by: Eduardo Bonet <ebonet@gitlab.com>
Approved-by: Jay Montal <jmontal@gitlab.com>
Approved-by: Jacques Erasmus <jerasmus@gitlab.com>
Approved-by: Bob Van Landuyt <bob@gitlab.com>
Co-authored-by: Gary Holtz <gholtz@gitlab.com>