Merge branch 'btn-sm-side-margin-variable' into 'master'

Remove btn-sm-side-margin misc variable in favor of gitlab ui

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126592



Merged-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Tomas Bulva <tbulva@gitlab.com>
Reviewed-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Co-authored-by: Taurie Davis <taurie@gitlab.com>