fix: unblock external Postgres pre-install hook

[sagarkt]

Summary

• let the external PostgreSQL database-creator hook use the namespace default service account
• remove its dependency on devtron-default-sa, which Helm creates only after pre-install hooks finish

Root cause

The database creator runs as a pre-install hook, while devtron-default-sa is a normal release resource. Kubernetes therefore rejects the hook pod before Helm has created that service account.

Validation

• helm lint with CICD, Argo CD, and external PostgreSQL enabled
• rendered the migrator template and verified the database creator is present without a release service-account dependency
• verified the database creator remains absent when external PostgreSQL is disabled

Closes #6973

[bito-code-review]

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