nuget: Bump the nunit group with 2 updates

Bumps NUnit from 3.13.3 to 4.4.0
Bumps NUnit3TestAdapter from 4.1.0 to 6.1.0

---
updated-dependencies:
- dependency-name: NUnit
  dependency-version: 4.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: nunit
- dependency-name: NUnit3TestAdapter
  dependency-version: 6.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: nunit
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Directory.Packages.props

@@ -25,8 +25,8 @@
     <PackageVersion Include="Microsoft.IO.RecyclableMemoryStream" Version="3.0.1" />
     <PackageVersion Include="MsgPack.Cli" Version="1.0.1" />
     <PackageVersion Include="NetCoreServer" Version="8.0.7" />
-    <PackageVersion Include="NUnit" Version="3.13.3" />
-    <PackageVersion Include="NUnit3TestAdapter" Version="4.1.0" />
+    <PackageVersion Include="NUnit" Version="4.4.0" />
+    <PackageVersion Include="NUnit3TestAdapter" Version="6.1.0" />
     <PackageVersion Include="open.nat.core" Version="2.1.0.5" />
     <PackageVersion Include="OpenTK.Core" Version="4.8.2" />
     <PackageVersion Include="OpenTK.Graphics" Version="4.8.2" />