Summary:
Add a Upgrade test for xCluster.

- Removed `xcluster_external_mini_cluster_base.cc` since it is not used.
- Added the capability to set `cluster_short_name` on ExternalMiniclusters. This enables us to add prefix to the log messages of the two xCluster cluster helping differentiate producer and consumer.
- Moved common xCluster test util functions to `xcluster_test_utils`.
- Added a helpful error in `xcluster_replication_group.cc` when the two cluster have AutoFlag conflicts.

Fixes #27522
Jira: DB-17081

Original commit: 5782612246b6a727333d34c01d59ae0ecd335f1e / D44477

Test Plan: xcluster_upgrade-itest

Reviewers: jhe, mlillibridge, slingam, #db-approvers

Reviewed By: slingam, #db-approvers

Subscribers: rthallam, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44659