Adds last_activity_on to Members table

What does this MR do and why?

Adds last_activity_on date column to the Members table.

Defaults to the current date, in order to pre-populate existing records.

The new column will be used to track the last activity time of members, which will be handled in [BE] Track member last activity time (#461338)

Refs #461336 (closed)

bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 125420, pg_backend_pid: 65784
main: == 20240520124019 AddLastActivityOnToMembers: migrating =======================
main: -- add_column(:members, :last_activity_on, :date, {:default=>#<Proc:0x000000015ef1d9c0 /gdk/gitlab/db/migrate/20240520124019_add_last_activity_on_to_members.rb:7 (lambda)>})
main:    -> 0.0014s
main: == 20240520124019 AddLastActivityOnToMembers: migrated (0.0052s) ==============

main: == [advisory_lock_connection] object_id: 125420, pg_backend_pid: 65784

bin/rails db:migrate:down:main VERSION=20240520124019
main: == [advisory_lock_connection] object_id: 124980, pg_backend_pid: 65369
main: == 20240520124019 AddLastActivityOnToMembers: reverting =======================
main: -- remove_column(:members, :last_activity_on, :date, {:default=>#<Proc:0x000000011f1362b0 /gdk/gitlab/db/migrate/20240520124019_add_last_activity_on_to_members.rb:7 (lambda)>})
main:    -> 0.0011s
main: == 20240520124019 AddLastActivityOnToMembers: reverted (0.0053s) ==============

main: == [advisory_lock_connection] object_id: 124980, pg_backend_pid: 65369

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.