fix: src/Bandwidth.Standard.Test/Bandwidth.Standard.Test.csproj to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DOTNET-NEWTONSOFTJSON-2774678

Author: snyk-bot

src/Bandwidth.Standard.Test/Bandwidth.Standard.Test.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.9.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.0" />
     <PackageReference Include="Moq" Version="4.20.69" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />