Merge branch '467417-cleanup-prominent-create-board-btn' into 'master'

Cleanup prominent_create_board_btn feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156324



Merged-by: Florie Guibert <fguibert@gitlab.com>
Approved-by: Lukas Wanko <lwanko@gitlab.com>
Approved-by: Rajan Mistry <rmistry@gitlab.com>
Approved-by: Aaron Huntsman <ahuntsman@gitlab.com>
Approved-by: Florie Guibert <fguibert@gitlab.com>
Co-authored-by: Serhii Yarynovskyi <syarynovskyi@gitlab.com>