Squeezebox grouping

[rajlaud]

Breaking change

This PR updates the squeezebox player synchronization framework to use the platform-level services media_player.join and media_player.unjoin. The list of synchronized players is now stored in the group_members state attribute.

Warn regarding depreciation of squeezebox.sync and squeezebox.unsync, which will be removed in two releases in favor of the new platform-level services.

The sync_group state attribute is deprecated in favor of group_members, and will be removed in two releases.

Proposed change

Bring the squeezebox sync framework up to date with current implementation of multi-room audio at the media_player platform level.

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: Update squeezebox.markdown home-assistant.io#22551

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.
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• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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[frenck]

The breaking change should mention the deprecation of the sync_group state attribute too (as it's replaced by the group_members attribute).

Otherwise: Tested locally and works as expected ✅