[Alerting] Upgrade to 7.10 without migrating old alerts will result in broken alerts

[gmmorris]

Following the rollback of the migration in 7.9 (#73420) we're now in a potential broken state when 7.10 is released.
We need to either revert the revert, or find another solution before 7.10 is released.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)