Add database prefer mode

What does this MR do?

Add database prefer mode

It is a boring alternative to Add support for container registry detabase pre... (!4635)

Change the default Registry database setting to... (!4672) sets the default registry.database.enabled to prefer.

Implement support for Container Registry's prefer mode with top-level checks:

• With external PostgreSQL + prefer mode, it rejects the settings, asks the user to enable/disable the database explicitly and review the docs
• With bundled PostgreSQL + prefer mode, it accepts the settings, passes the prefer mode to the sub-Chart

It also addes the followings to the NOTES:

• When prefer, warns about it, and relevant info of migration to explicit enabled and external db
• When disabled, notice about it, and recommendation to migrate.

A few changes to the check config:

• For registry database metrics and loadbalancing, the user must enable the metadata database explicity. It makes sense since those features require extra explicit settings.

Test plan

global:
  ingress:
    configureCertmanager: false
installCertmanager: false
registry:
  database:
    enabled: prefer
postgresql:
  install: false

helm template gitlab . -f registry.yaml  --show-only charts/registry/templates/configmap.yaml | tail -n 8

dabase.enabled	postgresql.install	Expected	Result
prefer	true	prefer	prefer
prefer	false	reject	reject
true	true	true	true
true	false	true	true
false	true	no database section	no database section
false	false	no database section	no database section

Related issues

Closes: #6172, #6183

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.