Expose information about what will not be migrated during GitLab migration

What does this MR do and why?

This MR exposes warning about what will not be migrated during GitLab migration Related to #345704 (closed)

Screenshots or screen recordings

How to set up and validate locally

0. Change the code to expect a higher GitLab version for project migration. For instance, change:

   # app/models/bulk_import.rb

   - MIN_MINOR_VERSION_FOR_PROJECT = 4 
   + MIN_MINOR_VERSION_FOR_PROJECT = 8

1. Open "New group" (/groups/new)
2. 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)
3. Observe alert
4. Revert change in app/models/bulk_import.rb
5. Refresh page
6. Observe alert is missing

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.