Remove migration configuration from charts

Context

This is a follow-up to Remove Migration Mode (#936 - closed), where the migration configuration was removed from the registry itself. We will need to remove the configuration from the Helm charts as well, as is the only place where the migration was added.

Problem

We are currently blocked by getting v3.73.1-gitlab to production before we can continue.

Implementation plan

1. Deploy v3.73.1-gitlab (or higher) to production
2. Update k8s-workloads to remove traces from the migration config section and secrets.
   1. pre
   2. gstg
   3. gprd
3. Update/rebase Draft: Registry: remove migration configuration (gitlab-org/charts/gitlab!3141 - closed) and get it merged.