`bbs2gh migrate-repo` should only require the Bitbucket username and password when it needs to talk to the Bitbucket API

As of #1057, we require `--bbs-server-url` to be provided for all variants of `bbs2gh migrate-repo` - even ones where we don't need to talk to the Bitbucket Server's API (i.e. the upload-and-import flow and the import-only flow).

We have started requiring the URL because we need it to communicate to the GitHub API which repo to pick from the archive when running the migration (see #1057 for more context).

This has the unwanted side effect that we now also require the Bitbucket Server username and password to be configured for all of the variants - even those ones that don't need to talk to the Bitbucket Server's API, and so won't use the username and password.

https://github.com/github/gh-gei/blob/d5bcc2b6c872517847a5dd857a99419baf334325/src/bbs2gh/Commands/MigrateRepo/MigrateRepoCommand.cs#L224-L231

We should improve this logic so that the Bitbucket username and password is only required for command variants which will talk to the Bitbucket API.