build(deps): bump Polyfill from 6.3.0 to 6.8.0

[dependabot[bot]]

Bumps Polyfill from 6.3.0 to 6.8.0.

Commits

• 6c72a76 add EnumPolyfill.IsDefined (#224)
• 3c74a55 Update Directory.Build.props
• 8907b2a add Stream DisposeAsync (#223)
• e42ecf1 expose Stream CopyToAsync when no ValueTask (#222)
• ea8be86 expose TextWriter Write StringBuilder when no memory ref (#221)
• 555c5ff add TextWriter.FlushAsync with CancellationToken (#220)
• 4efa9c9 refs or cleanup
• f880a68 use NETCoreSdkVersion instead of SdkAnalysisLevel
• f8faee0 respect LangVersion for preview features (#219)
• c4a0cf6 Update .gitignore
• Additional commits viewable in compare view

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[dependabot[bot]]

Superseded by #381.