chore(deps): update dependency polyfill to 9.21.0

[thomhurst]

This PR contains the following updates:

Package	Type	Update	Change
Polyfill	nuget	minor	9.20.0 → 9.21.0

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Release Notes

SimonCropp/Polyfill (Polyfill)

v9.21.0

Compare Source

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Configuration

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This PR has been generated by Renovate Bot.

[claude]

Dependency Update Review: Polyfill 9.20.0 → 9.21.0

This is a straightforward Renovate-generated minor version bump with a single-line change.

Change summary:

• Updates _TUnitPolyfillVersion in TUnit.Core/TUnit.Core.targets from 9.20.0 to 9.21.0
• The Polyfill package is used exclusively as a compile-time/analyzer-only dependency (IncludeAssets>compile; analyzers) with PrivateAssets>all, so there is no runtime impact

Notes:

• The central package version in Directory.Packages.props still pins Polyfill at 9.20.0. Since the .targets file uses VersionOverride for CPM projects, this won't break CPM consumers, but it's worth considering whether Directory.Packages.props should be kept in sync to avoid confusion for developers working on the TUnit repo itself.
• Polyfill is a source-only/analyzer package with no runtime surface, so minor bumps carry very low risk.
• Auto-merge is enabled, which is appropriate for this type of routine dependency maintenance.

Overall, the change looks correct and safe to merge.