Allow using existing ServiceAccount without creating a new one

The qdrant Helm chart currently does not provide a way to use an existing ServiceAccount without creating a new one. The chart's _helpers.tpl template serviceAccountName already supports this pattern through values, but:

1. The templates/serviceaccount.yaml template lacks a conditional guard and always creates a new ServiceAccount, regardless of serviceAccount.create setting.
2. The templates/statefulset.yaml hardcodes serviceAccountName: {{ include "qdrant.fullname" . }} instead of calling the qdrant.serviceAccountName helper, making it impossible to use a pre-existing ServiceAccount.

Current Behavior

1. serviceAccount.create: false + serviceAccount.name: existing-service acoount → Pod still tries to use auto-created SA; SCC/RBAC bindings for the existing SA are ignored
2. Users deploying in restricted environments cannot reuse centrally-managed ServiceAccounts
3. Duplicate ServiceAccount objects are created unnecessarily

Expected Behavior
When serviceAccount.create: false and serviceAccount.name is set:

1. No ServiceAccount resource should be created
2. Pod's serviceAccountName should use the specified existing SA
3. Existing RBAC/SCC bindings for that SA should apply to the pod

Inspect templates/serviceaccount.yaml and templates/statefulset.yaml, then compare their behavior with the qdrant.serviceAccountName helper in templates/_helpers.tpl. Render the chart with serviceAccount.create set to false and an existing name, and verify that no ServiceAccount is created and the workload uses the specified name.