feat(chart)!: change default service type to ClusterIP

[CatherineThompson]

default datahub-frontend and datahub-gms to ClusterIP for better security. this prevents unintentional public exposure on initial deployment.

BREAKING CHANGE: the default service type is no longer LoadBalancer. users requiring external access must now explicitly set service.type to LoadBalancer or configure an Ingress.

Closes #659

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

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Note

Medium Risk
Breaking Helm chart default: new installs will no longer create external LoadBalancer services for GMS/frontend, which can surprise users relying on automatic public exposure. Change is configuration-only but affects networking/accessibility on upgrade or fresh deploys.

Overview
Changes the default Kubernetes Service type for datahub-gms and datahub-frontend from LoadBalancer to ClusterIP, reducing accidental public exposure and requiring users to explicitly opt into LoadBalancer or use Ingress for external access.

Bumps the parent chart version to 0.9.0 and updates datahub-gms/datahub-frontend subchart versions to 0.4.0, plus updates the values reference and subchart READMEs to reflect the new defaults.

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