[BE] Add `last_activity_on` column to Members

Background

In [Gitlab.com] Automatic removal of dormant members (&7533) we intend to add a feature to identify and remove dormant users from Namespaces for GitLab.com

This is similar to the existing SM feature ( &5519 (closed)), but rather than deactivating users, we will remove members.

To achieve this, we will be adding a way to track member’s last activity timestamp within a namespace, so that we can identify dormant members and remove them if they've been dormant for greater than a predefined time period (e.g. 90 days).

Purpose

The purpose of this issue is to:

Add a column to the Members table that we'll use for tracking their last activity time

We can follow the same path that was taken for Users#last_activity_on for this, as it'll be used in much the same way:

• Create a migration to add a timestamp column to Members
  • no default value
• Pre-populate existing records with the current time (at time of migration). This will mean that when it comes to enabling the feature, we won't remove members immediately (i.e. because they have a nil value). This approach is not set in stone, we can take a different one if necessary. See #460840 (comment 1899076064) for more background/info around this decision.

The tracking of the activity will be handled in [BE] Track member last activity time (#461338 - closed)