Move "Allow users to request access" setting to a new subsection "Membership"

What does this MR do?

When feature flag saas_user_caps is enabled:

• It adds a new subsection "Membership" to group's settings under "Permissions, LFS, 2FA"
• It moves "Allow users to request access" item to be under the newly created subsection "Membership"

Screenshots

before	after

How to setup and validate locally (strongly suggested)

1. Enable the feature flag in rails console Feature.enable(:saas_user_caps)
2. Navigate to a group's settings (using a user that's a group owner)
3. Observe the changes in the "Permissions, LFS, 2FA" section

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • I haven't added a changed log, the change is under a feature flag
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #330029 (closed)