Update xknx to 0.21.2

[marvin-w]

Proposed change

Update xknx to version 0.21.2. Fixes some IP Secure bugs.

https://github.com/XKNX/xknx/releases/tag/0.21.2 (compare to get a diff to the last version)

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:

Checklist

• The code change is tested and works locally.
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• Tests have been added to verify that the new code works.

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  Updated and included derived files by running: python3 -m script.hassfest.
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  Updated by running python3 -m script.gen_requirements_all.
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[probot-home-assistant]

Hey there @Julius2342, @farmio, mind taking a look at this pull request as it has been labeled with an integration (knx) you are listed as a code owner for? Thanks!
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