Merge branch '430042-clean-up-apollo_boards-feature-flag-3' into 'master'

[Part 3] Clean up :apollo_boards feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/139622



Merged-by: Natalia Tepluhina <ntepluhina@gitlab.com>
Approved-by: Deepika Guliani <dguliani@gitlab.com>
Approved-by: Natalia Tepluhina <ntepluhina@gitlab.com>
Reviewed-by: Deepika Guliani <dguliani@gitlab.com>
Co-authored-by: Florie Guibert <fguibert@gitlab.com>