Add updated_at timestamp to PlanLimits

What does this MR do and why?

This MR adds the updated_at column to the plan_limits table for tracking when the records were last updated.

The column can be used for convenience with the recent addition of Admin Controls (https://gitlab.com/groups/gitlab-org/-/epics/9173) and also for caching and being able to generate a cache key from the timestamp.

Refs #419618 (closed)

Database info

up

 bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 224140, pg_backend_pid: 88317
main: == 20230725075011 AddUpdatedAtToPlanLimits: migrating =========================
main: -- add_column(:plan_limits, :updated_at, :datetime_with_timezone)
main:    -> 0.0015s
main: == 20230725075011 AddUpdatedAtToPlanLimits: migrated (0.0051s) ================

main: == [advisory_lock_connection] object_id: 224140, pg_backend_pid: 88317
ci: == [advisory_lock_connection] object_id: 224400, pg_backend_pid: 88319
ci: == 20230725075011 AddUpdatedAtToPlanLimits: migrating =========================
ci: -- add_column(:plan_limits, :updated_at, :datetime_with_timezone)
ci:    -> 0.0033s
ci: == 20230725075011 AddUpdatedAtToPlanLimits: migrated (0.0117s) ================

down

bin/rails db:migrate:down:main VERSION=20230725075011
main: == [advisory_lock_connection] object_id: 223860, pg_backend_pid: 89444
main: == 20230725075011 AddUpdatedAtToPlanLimits: reverting =========================
main: -- remove_column(:plan_limits, :updated_at, :datetime_with_timezone)
main:    -> 0.0014s
main: == 20230725075011 AddUpdatedAtToPlanLimits: reverted (0.0073s) ================

main: == [advisory_lock_connection] object_id: 223860, pg_backend_pid: 89444

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