Alerting: Keep legacy alert rule maxDataPoints and intervalMs during migration

[JacobsonMT]

What is this feature?

This feature retains legacy values for maxDataPoints and min interval (intervalMs) during legacy -> UA migration.

Why do we need this feature?

Currently, the legacy migration does not retain these two fields. This means that legacy alert rules which were running with maxDataPoints=1500 and intervalMs=(some value determined by time range and 1500, approximately time range / 1500) will, after migration, run with maxDataPoints=43200 and intervalMs=1000. This has two main drawbacks:

1. The UA query is different than the legacy query and might cause nuanced differences in results.
2. The old query with the new limits can cause errors if requesting too much data from the datasource. For example, a legacy prometheus query running over the last 4h once migrated would result in exceeded maximum resolution of 11,000 points per timeseries.

Who is this feature for?

Users migrating from legacy to UA.

Which issue(s) does this PR fix?:

Fixes #71988

Special notes for your reviewer:

Should not be merged without #71986 as users would not be able see or edit the migrated intervalMs Merged.

[grobinson-grafana]

Tested this and can see that 1500 and min_interval are migrated to the new rule.

[yuri-tceretian]

LGTM