Add mV as a unit for voltage and enable conversions

[jbouwh]

Proposed change

The unit mV was added for device_class voltage, but only V was still accepted as valid unit.
Further conversion support between V and mV (voltage) was missing.

Related frontend PR: home-assistant/frontend#14921

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 fixes or closes issue: fixes Entity xxx_voltage is using native unit of measurement 'mV' which is not a valid unit for the device class ('voltage') #84762
• This PR is related to issue:
• Link to documentation pull request:
  Add mV unit to number and sensor entity documentation developers.home-assistant#1598

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[home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (sensor) you are listed as a code owner for? Thanks!

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[epenet]

See #84492

I suggest that you restrict this PR to only electric potential, and create the corresponding:

• arch discussion
• dev docs PR
• docs PR
• frontend PR

[epenet]

Also, please note that the way to "fix" this in 2023.1 beta is to remove the incorrect device class.
Then it can be added again for 2023.2 once the conversion has been added.

[jbouwh]

• dev docs PR
• docs PR
• frontend PR
• architecture discussion Make mV an officially supported unit of voltage architecture#851, based on Make mA an officially supported unit of current architecture#847 The same situation exists for the mV unit. It was added as a constant but never accepted as a change.

[frenck]

Thanks, @jbouwh! ✅

As I wrote in the linked architectural discussion, we are going to pull this into the current 2023.1 beta, to minimize the impact of the new checks.

../Frenck

Comments have been addressed