Gitea migration should migrate Pull Requests, not Merge Requests

[delvh]

• Gitea version (or commit ref): all up to 1.16.0+dev-138-gdb1e3d02a

Description

Gitea does not use the term Merge Request.
It uses Pull Request. Yet, when migrating a gitea repo, we are presented with migrating Merge Requests.
Fix should be a one liner.

Screenshots

[delvh]

I can fix that as part of #16781 if wanted.