Move Members menu item to group information menu

What does this MR do?

In this MR we move the Members menu, to the Group overview (or Group information) menu.

At the moment it looks like:

All these changes are behind the feature flag sidebar_refactor.

Screenshots (strongly suggested)

After the MR, the new menu item will be:

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• 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.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #224701 (closed)