Merge branch '474594-remove-mixin-btn-red-from-buttons-scss' into 'master'

Replace only instance of btn-red with contents of btn-red

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167733



Merged-by: Scott de Jonge <sdejonge@gitlab.com>
Approved-by: Scott de Jonge <sdejonge@gitlab.com>
Co-authored-by: James Rushford <jrushford@gitlab.com>