Bump TUnit from 1.21.6 to 1.21.20 (#951)

Updated [TUnit](https://github.com/thomhurst/TUnit) from 1.21.6 to
1.21.20.

<details>
<summary>Release notes</summary>

_Sourced from [TUnit's
releases](https://github.com/thomhurst/TUnit/releases)._

## 1.21.20

<!-- Release notes generated using configuration in .github/release.yml
at v1.21.20 -->

## What's Changed
### Other Changes
* fix: respect TUnitImplicitUsings set in Directory.Build.props by
@​thomhurst in thomhurst/TUnit#5225
* feat: covariant assertions for interfaces and non-sealed classes by
@​thomhurst in thomhurst/TUnit#5226
* feat: support string-to-parseable type conversions in [Arguments] by
@​thomhurst in thomhurst/TUnit#5227
* feat: add string length range assertions by @​thomhurst in
thomhurst/TUnit#4935
* Fix BeforeEvery/AfterEvery hooks for Class and Assembly not being
executed by @​Copilot in thomhurst/TUnit#5239
### Dependencies
* chore(deps): update tunit to 1.21.6 by @​thomhurst in
thomhurst/TUnit#5228
* chore(deps): update dependency gitversion.msbuild to 6.7.0 by
@​thomhurst in thomhurst/TUnit#5229
* chore(deps): update dependency gitversion.tool to v6.7.0 by
@​thomhurst in thomhurst/TUnit#5230
* chore(deps): update aspire to 13.2.0 - autoclosed by @​thomhurst in
thomhurst/TUnit#5232
* chore(deps): update dependency typescript to v6 by @​thomhurst in
thomhurst/TUnit#5233
* chore(deps): update dependency polyfill to 9.23.0 by @​thomhurst in
thomhurst/TUnit#5235
* chore(deps): update dependency polyfill to 9.23.0 by @​thomhurst in
thomhurst/TUnit#5236


**Full Changelog**:
thomhurst/TUnit@v1.21.6...v1.21.20

Commits viewable in [compare
view](thomhurst/TUnit@v1.21.6...v1.21.20).
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Directory.Packages.props

@@ -24,7 +24,7 @@
     <PackageVersion Include="Azure.Identity" Version="1.19.0" />
     <PackageVersion Include="Azure.Monitor.OpenTelemetry.AspNetCore" Version="1.4.0" />
     <PackageVersion Include="Microsoft.ApplicationInsights.Profiler.AspNetCore" Version="3.0.1" />
-    <PackageVersion Include="TUnit" Version="1.21.6" />
+    <PackageVersion Include="TUnit" Version="1.21.20" />
     <PackageVersion Include="EssentialCSharp.Shared.Models" Version="$(ToolingPackagesVersion)" />
     <PackageVersion Include="HtmlAgilityPack" Version="1.12.4" />
     <PackageVersion Include="IntelliTect.Multitool" Version="2.0.0" />