[Feature flag] Make Group SCIM generally available

What

Remove the :group_scim feature flag after selectively trialling it with individual groups.

Steps

• Test SCIM on staging with test group
• Test SCIM on GitLab.com with test group
• Remove the feature flag SCIM documentation (from both https://docs.gitlab.com/ee/api/scim.html and https://docs.gitlab.com/ee/user/group/saml_sso/scim_setup.html)
• Test with beta customers on GitLab.com
• Increase coverage of QA/feature tests
• Security review
• Security fixes merged if needed
• Remove feature flag and add changelog entry