update NUnit to 3.14.0

Author: janusw

src/GeoJSON.Net.Tests/GeoJSON.Net.Tests.csproj

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\GeoJSON.Net\GeoJSON.Net.csproj" />
-    <PackageReference Include="NUnit" Version="3.7.1" />
+    <PackageReference Include="NUnit" Version="3.14.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
   </ItemGroup>