Bump Argon, NUnit and Verify.NUnit in /src (#275)

Bumps [Argon](https://github.com/SimonCropp/Argon), [NUnit](https://github.com/nunit/nunit) and [Verify.NUnit](https://github.com/VerifyTests/Verify). These dependencies needed to be updated together.

Updates `Argon` from 0.26.0 to 0.26.0
- [Release notes](https://github.com/SimonCropp/Argon/releases)
- [Commits](SimonCropp/Argon@0.26.0...0.26.0)

Updates `NUnit` from 4.3.0 to 4.3.0
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/main/CHANGES.md)
- [Commits](nunit/nunit@4.3.0...4.3.0)

Updates `Verify.NUnit` from 28.6.1 to 28.7.0
- [Release notes](https://github.com/VerifyTests/Verify/releases)
- [Commits](VerifyTests/Verify@28.6.1...28.7.0)

---
updated-dependencies:
- dependency-name: Argon
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: NUnit
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: Verify.NUnit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/Directory.Packages.props

@@ -14,7 +14,7 @@
     <PackageVersion Include="System.Net.Http" Version="4.3.4" />
     <PackageVersion Include="System.Text.Json" Version="9.0.0" />
     <PackageVersion Include="Verify.DiffPlex" Version="3.1.2" />
-    <PackageVersion Include="Verify.NUnit" Version="28.6.1" />
+    <PackageVersion Include="Verify.NUnit" Version="28.7.0" />
     <PackageVersion Include="Verify.Http" Version="6.4.1" />
     <PackageVersion Include="NUnit" Version="4.3.0" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />