Add a prefix to SCIM tokens behind a feature flag

Prefixes SCIM OAuth Access Tokens with `glsoat-` following the
guidance at
https://docs.gitlab.com/ee/development/secure_coding_guidelines.html#token-prefixes.

GitLab applies a prefix to some of its generated secrets. For example, a
Personal Access Token begins with `glpat-`. This MR adds a prefix to
SCIM Tokens. It also updates our frontend secret detection which
helps prevent users from leaking tokens via Issue / MR comments.

SCIM tokens can belong to a Group, or have no Group and be an instance
token. These tokens are used to authenticate against the APIs described
at https://docs.gitlab.com/ee/development/internal_api/#group-scim-api
and
https://docs.gitlab.com/ee/development/internal_api/#instance-scim-api
respectively.

A feature flag is being used to reduce the risk of breaking third-party
integrations, which might have made assumptions about the format of
GitLab's SCIM tokens remaining static.
See https://gitlab.com/gitlab-org/gitlab/-/issues/435096#note_1691498327
for discussion.

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/435096

Changelog: changed
EE: true