Deprecate HomeKit auto start

[frenck]

Breaking change

The HomeKit auto_start configuration option has been deprecated. It dates from the "old"-days to prevent entities not being available yet when HomeKit started. However, nowadays, Home Assistant ensures all entities are available on startup instantly.

Therefore, this option is no longer needed. If you still have it in your configuration (and have automations for it to start HomeKit); those can be safely removed.

Proposed change

Basically, see the breaking change described above. This PR doesn't change anything, besides marking it deprecated.

It more than often causes questions; or I see people still thinking it is needed. By marking it deprecated, it will be clear.

Docs PR: home-assistant/home-assistant.io#16874

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

Example entry for configuration.yaml:

# Example configuration.yaml
homekit:
  - autostart: false

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @bdraco, mind taking a look at this pull request as its been labeled with an integration (homekit) you are listed as a codeowner for? Thanks!
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