Move group "Administration" nav behind feature flag

What does this MR do?

This is a follow up to !28057 (merged). There has been some negative feedback around this change (see thread here: #209020 (closed)) and it probably should have been behind a feature flag from the get go (learning experience 🙂). Because of this we have decided to move this behind a feature flag until we can best determine how to proceed.

Changes

• Moves group "Administration" nav behind group_administration_nav_item feature flag
• Reverts documentation updates made in !28057 (merged). To be further discussed in #213726 (closed)

Local Testing

1. Install an EE license. See https://about.gitlab.com/handbook/developer-onboarding/#working-on-gitlab-ee.
2. Enable these SAML SSO related feature flags: group_saml, enforced_sso, and enforced_sso_requires_session
3. Enable group_saml in config/gitlab.yml. See https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/howto/saml.md#gitlab-configuration. Note: Ensure this is added to the development: section of your config
4. Enable group_administration_nav_item feature flag
   • bin/rails console
   • group = Group.find_by(name: 'group name')
   • Feature.enable(:group_administration_nav_item, group)

Screenshots

Page	Before !28057 (merged) was merged	After w/ feature disabled	After w/ feature enabled
Parent Group
Subgroup

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