Update helm-dashboard.serviceAccountName Template Logic in _helpers.tpl

[unitmatrix]

This pull request updates the helm-dashboard.serviceAccountName template logic in the _helpers.tpl file to improve the handling of service account names. The changes include:

• Utilizing the default function to set the service account name based on the serviceAccount.create value.
• Ensuring that the service account name is generated using the helm-dashboard.fullname template when serviceAccount.create is true, otherwise defaulting to "default".
• Maintaining backward compatibility with existing configurations by preserving the use of .Values.serviceAccount.name.

These changes enhance the flexibility and robustness of the Helm chart's service account naming conventions.