Remove 'ai_access_token' column from AppSettings: 16.9 - new migration

This is part of the Corrective action: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24523+

In %16.5 we:

• added correct ignore rule for encrypted ai_access_token columns
• marked RemoveApplicationSettingsAiAccessTokenColumn as no-op

We need a new migration to actually remove 'ai_access_token' columns, in %16.6

UPD: we want to wait 2-3 more releases on top of formal requirement of +1 release to :ignore_column, see !134770 (comment 1617390073)