Show what data will not be migrated during GitLab migration

#341422 (closed) introduced backend API to report about migration limitations when connecting to instance, running old version of GitLab

This issue is about exposing this information to end-user in friendly way. I'm basing my proposal on following assumptions:

• User by-default assuming that everything is migrated. We are providing explicit link to docs to define what "everything" means. This issue is not about communicating what will be migrated
• We don't want to annoy users with extra warnings
• We need to clearly communicate why limitations are in effect
• Since there are two instances of GitLab (source & target) involved we need to clearly communicate which one is problematic :)

My proposal:

Suggested wording:

Title: https://gitlab.example.com is running outdated GitLab version (v.14.2.0)

Following data will not be migrated: 
* projects (require v14.4.0)
* some-other-stuff (require v14.3.0)

Contact system administrator of https://gitlab.example.com to upgrade GitLab if you need this data in your migration

WDYT @hdelalic?

/cc @wortschi