[Feature flag] Make signing into GitLab.com with SAML generally available

What

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

Steps

• Test sign in flows on staging with test group
• Test sign in flows on GitLab.com with test group
• Update and improve Group SAML documentation
• 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