Migrate system_note_metadata.id to bigint

system_note_metadata.id column is an int4 primary key currently exceeding 50% of saturation. As a result, that key must be migrated to a big int to ensure we don't exhaust potential values.

Capacity planning issue - https://gitlab.com/gitlab-com/gl-infra/capacity-planning/-/issues/1173.

Timeline

This process needs to be started ASAP - exhausting this column would cause serious harm to the running application.

Process

Following the process outlined here, we must: