Remove feature flag from invite members banner [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes feature flag invite_your_teammates_banner_a and makes invite members banner permanent part of the feature on group overview page.

It is displayed on the Group Overview page when:

• page viewer can admin group

• group has 1 or fewer members including yourself

• the group was not just created, so there is no "Group successfully created" flash message also on the page in that spot.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/323106

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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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

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Related to #323106