Adds a feature flag (alertingUpgradeDryrunOnStart) that will dry-run the legacy
alert upgrade on startup. It is enabled by default.
When on legacy alerting, this feature flag will log the results of the legacy
alerting upgrade on startup and draw attention to anything in the current legacy
alerting configuration that will cause issues when the upgrade is eventually
performed. It acts as a log warning for those where action is required before
upgrading to Grafana v11 where legacy alerting will be removed.