Fix Growatt integration authentication error for legacy config entries

[johanzander]

Adds migration logic to handle config entries created before the CONF_AUTH_TYPE field was introduced in PR home-assistant#149783. The migration automatically detects the authentication type based on the presence of either CONF_TOKEN (for API token auth) or CONF_USERNAME/CONF_PASSWORD (for password auth) and updates the config entry accordingly.

This resolves the "Unknown authentication type in config entry" error that users experienced after upgrading to Home Assistant Core 2025.11.0.

Fixes home-assistant#155906

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