Implement automatic migration for Growatt Server DEFAULT_PLANT_ID entries

[johanzander]

Proposed change

Implements automatic config entry migration for Growatt Server integration to handle legacy entries with DEFAULT_PLANT_ID ("0").

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• [] Code quality improvements to existing code or addition of tests

Additional information

• This PR addresses review feedback from @joostlek in Increase Growatt Server test coverage to 97% #159544 (review).

Checklist

• I understand the code I am submitting and can explain how it works.
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• Tests have been added to verify that the new code works.
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If user exposed functionality or configuration variables are added/changed:

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To help with the load of incoming pull requests:

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[joostlek]

Can we bump the version of the config entry (I believe this is backwards compatible so we can do a minor version) and do this in async_migrate_entry?