Bump TUnit from 1.18.9 to 1.18.21 (#894)

Updated [TUnit](https://github.com/thomhurst/TUnit) from 1.18.9 to
1.18.21.

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

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

## 1.18.21

<!-- Release notes generated using configuration in .github/release.yml
at v1.18.21 -->

## What's Changed
### Other Changes
* avoid some string alloc by using AppendJoin by @​SimonCropp in
thomhurst/TUnit#4971
* respect attribute namespace during migration by @​SimonCropp in
thomhurst/TUnit#5066
* fix: parent data source initialization spans under session span by
@​thomhurst in thomhurst/TUnit#5071
### Dependencies
* chore(deps): update tunit to 1.18.9 by @​thomhurst in
thomhurst/TUnit#5060
* chore(deps): update dependency polyfill to 9.14.0 by @​thomhurst in
thomhurst/TUnit#5063
* chore(deps): update dependency
microsoft.testing.extensions.codecoverage to 18.5.2 by @​thomhurst in
thomhurst/TUnit#5065
* chore(deps): update dependency polyfill to 9.14.0 by @​thomhurst in
thomhurst/TUnit#5062
* chore(deps): update actions/github-script action to v8 by @​thomhurst
in thomhurst/TUnit#5053
* chore(deps): update dependency polyfill to 9.15.0 by @​thomhurst in
thomhurst/TUnit#5067
* chore(deps): update dependency polyfill to 9.15.0 by @​thomhurst in
thomhurst/TUnit#5068


**Full Changelog**:
thomhurst/TUnit@v1.18.9...v1.18.21

Commits viewable in [compare
view](thomhurst/TUnit@v1.18.9...v1.18.21).
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Directory.Packages.props

@@ -21,7 +21,7 @@
     <PackageVersion Include="Azure.Identity" Version="1.18.0" />
     <PackageVersion Include="Azure.Monitor.OpenTelemetry.AspNetCore" Version="1.4.0" />
     <PackageVersion Include="Microsoft.ApplicationInsights.Profiler.AspNetCore" Version="3.0.1" />
-    <PackageVersion Include="TUnit" Version="1.18.9" />
+    <PackageVersion Include="TUnit" Version="1.18.21" />
     <PackageVersion Include="EssentialCSharp.Shared.Models" Version="$(ToolingPackagesVersion)" />
     <PackageVersion Include="HtmlAgilityPack" Version="1.12.4" />
     <PackageVersion Include="IntelliTect.Multitool" Version="2.0.0" />