Move Members menu item to Project information menu

What does this MR do?

In this MR, we're moving (behind a feature flag) the Members menu in the project sidebar, to the Project information menu.

Screenshots (strongly suggested)

After the MR the menu looks like this:

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)