Allow users to autocomplete members of private group via group share

What does this MR do and why?

Allow users to autocomplete members of private group via group share

Related issue: #36069 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Create a public project
2. Go to Members page. Invite a group, select a private group
3. Add some users as a member of the private group (say bob)
4. Login, or impersonate as a member of the public project
5. Create a new issue in the project. Try to autocomplete @bob in the issue description.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.