Crons: Backfill monitor with default environments

[evanpurkhiser]

All old monitor checkins need to be associated to some environment, this should be the same default environment that we're using for new checkins added in #45013.

The status should be whatever the existing status of the monitor is.

1. Create environments for all monitors that don't already have an environment. The status should be whatever the existing status is for the monitor.
2. Update all checkins to associate the checkin to the monitor environment, there are a lot of rows here so @wedamija has some prior art for high volume backfills.