Make async_step_discovery default for MQTT config flows

[emontnemery]

Proposed change

Make async_step_mqtt default to async_step_discovery for config flows

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)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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• The code change is tested and works locally.
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[MartinHjelmare]

Looks good!