Update gitlab migration and github importers dropdown to use graphql

What does this MR do and why?

As a part of Update all import-related group selection dropd... (#375958 - closed) we're unifying all our group selection dropdowns to use single source of truth - specifically GraphQL This MR updates all project-based importers which use _githubish partial (it's almost all of them) and also gitlab migration feature to use dropdown

Screenshots or screen recordings

GitLab Migration

GitHub

How to set up and validate locally

GitLab Migration

• Open "New group" -> Import (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Open any dropdown and see available groups for user

GitHub

• Generate new GitHub token at https://github.com/settings/tokens
• open /import/github/new and use that token
• Open any dropdown and see available groups for user

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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