Draft: Removes Members.last_activity_on default value

What does this MR do and why?

Initially when adding the last_activity_on column to the Members table, we added a default value so that it was auto-populated until we had implemented the tracking that actually populates the value.

In #461338 we implemented tracking for Member activity, so we no longer need (or want) the default value.

This MR removes the default.

Refs #462751

Database details

migrate up

bin/rails db:migrate                                 

main: == [advisory_lock_connection] object_id: 128580, pg_backend_pid: 90819
main: == 20240701094837 RemoveMembersLastActivityOnDefaultValue: migrating ==========
main: -- change_column_default(:members, :last_activity_on, {:to=>nil})
main:    -> 0.0157s
main: == 20240701094837 RemoveMembersLastActivityOnDefaultValue: migrated (0.0196s) =

migrate down

bin/rails db:migrate:down:main VERSION=20240701094837
main: == [advisory_lock_connection] object_id: 128100, pg_backend_pid: 91278
main: == 20240701094837 RemoveMembersLastActivityOnDefaultValue: reverting ==========
main: -- change_column_default(:members, :last_activity_on, #<Proc:0x000000030ee7a5c0 /code/gdk/gitlab/db/post_migrate/20240701094837_remove_members_last_activity_on_default_value.rb:14 (lambda)>)
main:    -> 0.0415s
main: == 20240701094837 RemoveMembersLastActivityOnDefaultValue: reverted (0.0459s) =

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