4bb2ba53e8
After migrating to unified alerting, users must explicitly allow rolling
back to legacy alerting by setting force_migration = true in config.
This updates the panic message to clarify why that's required and what
the consequences of rolling back will be.
Fixes #50469
(cherry picked from commit 30f035ca34)
Co-authored-by: Joe Blubaugh <joe.blubaugh@grafana.com>