Merge branch '333011-remove-advanced-search-multi-project-select-feature-flag' into 'master'

Remove advanced_search_multi_project_select feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/178134



Merged-by: Pam Artiaga <partiaga@gitlab.com>
Approved-by: Safwan Ahmed <safahmed@gitlab.com>
Approved-by: Pam Artiaga <partiaga@gitlab.com>
Reviewed-by: Safwan Ahmed <safahmed@gitlab.com>
Co-authored-by: Arturo Herrero <arturo.herrero@gmail.com>