Use My Home Assistant for OAuth2 redirect callbacks

[frenck]

Breaking change

Home Assistant will now use My Home Assistant redirecting OAuth2 callback over. If you need to re-authenticate with an exiting OAuth2 application in the future, you might need to adjust the external application configuration. Please check the documentation of the specific integration on how to configure this.

Proposed change

Use https://my.home-assistant.io/redirect/callback as the redirect URI when the my integration is enabled.

Needs:

• Simplify OAuth2 authorize callback URL my.home-assistant.io#240
• Simplify OAuth2 authorize callback URL frontend#12765

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

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Use My Home Assistant for OAuth2 redirect callbacks home-assistant.io#22878

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.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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