Fix Shelly button unique_id migration

[bieniu]

Breaking change

Proposed change

Fix Shelly button migration to avoid duplicate unique_id.

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 Reboot buttons for Shelly devices is trying to use a unique_id that's already in use. #95560
• This PR is related to issue:
• Link to documentation pull request:

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
• I have followed the perfect PR recommendations
• 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

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• 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.
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• Untested files have been added to .coveragerc.

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

[home-assistant]

Hey there @balloob, @thecode, @chemelli74, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (shelly) you are listed as a code owner for? Thanks!

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[bieniu]

Thanks @thecode