Fix stale user highest role stats on dormant user deactivation

What does this MR do and why?

This MR fixes #369393 (closed) by updating the dormant user deactivation worker to execute user deactivations via the User model instead of running a SQL query.

A post-deployment migration is included to remove any User Highest Role entries that belong to deactivated users. After execution of the migration, the Users Statistics page may still show inaccurate data until the CreateStatisticsWorker job runs.

Migrations

This MR includes the following migration: RemoveDeactivatedUserHighestRoleStats - removes entries in the user_highest_roles that belong to deactivated users. This data is normally deleted when a user is deactivated, but they were not being deleted when users were deactivated via the dormant user deactivation feature.

There is no revert action in this migration.

UP

main: == 20220802204737 RemoveDeactivatedUserHighestRoleStats: migrating ============
main: == 20220802204737 RemoveDeactivatedUserHighestRoleStats: migrated (0.0411s) ===

DOWN

main: == 20220802204737 RemoveDeactivatedUserHighestRoleStats: reverting ============
main: == 20220802204737 RemoveDeactivatedUserHighestRoleStats: reverted (0.0014s) ===

Screenshots or screen recordings

See #369393 (closed)

The counts in the Users Statistics screenshot are corrected by this MR to reflect only active users. There are no UI changes.

How to set up and validate locally

See steps to reproduce in #369393 (closed)

In Deactivate users via the dormant user deactivation feature step 8, this MR will change the outcome by updating the Users Statistics page counts.

MR acceptance checklist

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