Bump NUnit from 4.0.1 to 4.5.1

---
updated-dependencies:
- dependency-name: NUnit
  dependency-version: 4.5.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

GithubToc.Tests/GithubToc.Tests.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="IDisposableAnalyzers" Version="4.0.7" PrivateAssets="all" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="Microsoft.VisualStudio.Threading.Analyzers" Version="17.8.14" PrivateAssets="all" />
-    <PackageReference Include="NUnit" Version="4.0.1" />
+    <PackageReference Include="NUnit" Version="4.5.1" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="ReflectionAnalyzers" Version="0.3.1" PrivateAssets="all" />
     <PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.556" PrivateAssets="all" />