Migrate switch device classes to StrEnum

[frenck]

Breaking change

This PR deprecates the use of the DEVICE_CLASS_* constants and DEVICE_CLASSES constant for switch entities. Use the SwitchDeviceClass enum instead.

Proposed change

Replaces the device class constants of the switch platform with a StrEnum.

The old CONST is currently still accepted and working (backward compatible). This gives core and custom integration time to adjust.

At this point, I've not set a deprecation period as it is hard to indicate how long this would take.

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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[probot-home-assistant]

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

Strictly speaking a mere deprecation is not yet a breaking change. Do we want to highlight this as such as a kind of a warning anyway?

It's unfortunate if we don't have anywhere to put a deprecation warning about using the old constants, with just a comment it's likely that few people will notice. Then again labeling as breaking change already now could help some.

[frenck]

Strictly speaking a mere deprecation is not yet a breaking change.

We generally mark all deprecation as breaking; As that makes it show up when we compile release notes at that point we can decide what to do.