Adds last_enforced_at column to NamespaceLimits

What does this MR do and why?

Adds last_enforced_at column to NamespaceLimits

In order to track when a root namespace last had storage enforcement apply to the namespace, this adds the column and associated logic that will update the timestamp once per day.

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/382200

bin/rails db:migrate

main: == [advisory_lock_connection] object_id: 223520, pg_backend_pid: 56819
main: == 20230629071427 AddLastEnforcedAtToNamespaceLimits: migrating ===============
main: -- add_column(:namespace_limits, :last_enforced_at, :datetime_with_timezone)
main:    -> 0.0018s
main: == 20230629071427 AddLastEnforcedAtToNamespaceLimits: migrated (0.0061s) ======

bin/rails db:rollback:main
main: == [advisory_lock_connection] object_id: 223280, pg_backend_pid: 56346
main: == 20230629071427 AddLastEnforcedAtToNamespaceLimits: reverting ===============
main: -- remove_column(:namespace_limits, :last_enforced_at, :datetime_with_timezone)
main:    -> 0.0019s
main: == 20230629071427 AddLastEnforcedAtToNamespaceLimits: reverted (0.0077s) ======

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