Migration to remove gitlab as import_source

What does this MR do and why?

The Gitlab.com importer (aka gitlab) is no longer supported and has been removed as a possible import source from the codebase in !118891 (merged).

This adds a migration to remove gitlab as an import source from the database table for application settings.

main: == 20230504084524 RemoveGitlabImportSource: migrating =========================
main: == 20230504084524 RemoveGitlabImportSource: migrated (0.3210s) ================

It also removes "Gitlab.com" from the Omni auth help text in the Admin UI

Screenshots

Before

After

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.

Related to #388246 (closed)