Merge branch '503073-migrate-gl-border-gray-50-gl-border-subtle' into 'master'

Replace gl-border-gray-50 with gl-border-subtle

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173948



Merged-by: Himanshu Kapoor <info@fleon.org>
Approved-by: Sascha Eggenberger <seggenberger@gitlab.com>
Approved-by: Chen Charnolevsky <ccharnolevsky@gitlab.com>
Approved-by: Himanshu Kapoor <info@fleon.org>
Co-authored-by: sdejonge <sdejonge@gitlab.com>