Clarify required permission for GitLab Migration

What does this MR do and why?

Adds hints to UI and docs for informing users that they need to have Owner permission to migrate their groups.

Related to #361170 (closed).

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How to set up and validate locally

Go to Menu->Groups->Create group.

Choose Import group.

Enter the URL of GitLab instance where you have (or don't have for testing behavior of empty table) a group with Owner permission and valid Personal access token.

Then click Connect instance.

MR acceptance checklist

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