[Feature flag] Enable Group SAML metadata output

What

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

This feature provides metadata to help when configuring SAML Identity Providers.

This wasn't a high priority feature, but gives us some room to experiment with group level feature flags in the Manage team. In particular we can use this to prepare for https://gitlab.com/gitlab-org/gitlab-ee/issues/9654 by enabling the flag for a single group on GitLab.com now. This is slightly more complicated than other feature flags because enabling per group can't be done through chatops.

Steps

• Use test group on GitLab.com to experiment with metadata configuration
• Update and improve Group SAML documentation
• [Optional] Test with a beta customer on GitLab.com
• Remove feature flag and add changelog entry