Well segment: Change border color to default gray-100

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/147674



Merged-by: Paul Slaughter <pslaughter@gitlab.com>
Approved-by: Chad Lavimoniere <clavimoniere@gitlab.com>
Approved-by: Jay Montal <jmontal@gitlab.com>
Approved-by: Paul Slaughter <pslaughter@gitlab.com>
Co-authored-by: Sascha Eggenberger <seggenberger@gitlab.com>