Sync groups on sign-in for GitLab.com Group SAML

What does this MR do?

Related to #267020 (closed).

The last MR, which will enable Group SAML Group Sync by default. 🙌

This MR contains a few smaller pieces that are needed to default-enable this feature. The feature was not enabled previously so there's no concern about causing breaking changes or users being adversely affected.

• Minor fix to policies to ensure SAML Group Links can be managed on subgroups where SAML is enabled at the top-level group.
• Moves the feature from Ultimate/Gold to Premium/Silver per latest discussion at #267020 (comment 441825784)
• Default-enables the SAML Group Links feature flag.
• Adds documentation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team