koji_tag: use editTagExternalRepo

[ktdreyer]

If an external repository is a wrong priority or merge_mode for a tag, the koji_tag Ansible module removes the repo and then re-adds it with the correct settings. We did this to work around a bug in the editTagExternalRepo RPC, https://pagure.io/koji/issue/1857

The editTagExternalRepo RPC is working now in Koji v1.21. We should be able to use editTagExternalRepo now if the hub is running that version.

To avoid hitting the bug on old Koji hubs, we should always pass the merge_mode kwarg to editTagExternalRepo. This will cause old/broken Koji hubs to raise an error, rather than silently wiping out a non-"koji" merge_mode.

The purpose of this change is to shrink race conditions where the repos are incorrect, and apply large changes faster.

[ktdreyer]

This is pretty low-priority for me relative to the other tickets.

We would be shrinking (not eliminating) the race condition here, because the hub's edit_tag_external_repo() method simply calls remove_external_repo_from_tag() and then add_external_repo_to_tag() internally. There is still a small window where the repo(s) are missing from the tag.

It is going to add more complexity to provide backwards-compatibility for hubs prior to v1.21.

Not going to discourage anyone else from working on this... :)