Create Helm release to execute Container Registry post deployment migrations

Problem statement

Post deployment migrations (PDMs) for the Container Registry metadata database on production, staging and preprod are currently executed manually. In this epic (gitlab-org&17933), we are automating the execution of PDMs of Container Registry.

We want to automate the execution of Container Registry PDMs using the gitlab Helm chart and registry sub-chart, and a Helm release. Refer to #21188 (comment 2583225657) for a comparison of using the existing Helm release or a new one.

Proposal

• Create a new Helm release for executing Container Registry migrations
• Disable Container Registry deployment and other objects in the new release. The mechanism for doing this is being added in #21257 (closed).
• Enable the Container Registry migrations job
• Set SKIP_POST_DEPLOYMENT_MIGRATIONS to false
• Execute PDMs by connecting to the regional cluster in the main stage of staging and production