Only the plain git migration can clone from a local server path. Other
services (Gitea, GitLab, Gogs, OneDev, GitBucket, Codebase, CodeCommit,
Bitbucket Server) are accessed through their remote API, so a local path
never works, yet their migrate forms advertised "or a local server
path".
- Remove the local path hint from those templates.
- Add `IsMigrateURLAllowedForService` and use it in the web and API
migrate handlers and in `MigrateRepository`, rejecting local paths for
any service other than plain git.
- Add a unit test.
Raised in review of #39913.
_Authored with Codet (AI assistant) on behalf of @lunny._
---------
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
