Update to latest .NET versions (#37)

Test and Benchmark projects now target .NET Core 3.1. Library now targets .NET Standard 2.1 (and continues to target .NET Standard 2.0 and .NET Core App 2.1).

Solutions/Ais.Net.Benchmarks/Ais.Net.Benchmarks.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <Import Project="..\Common.NetCore_2_2.proj" />
+  <Import Project="..\Common.NetCore_3_1.proj" />
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>

Solutions/Ais.Net.Specs/Ais.Net.Specs.csproj

@@ -1,27 +1,34 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <Import Project="..\Common.NetCore_2_2.proj" />
+  <Import Project="..\Common.NetCore_3_1.proj" />
 
   <PropertyGroup>
 
     <IsPackable>false</IsPackable>
-    <NoWarn>RCS1029;RCS1089;SA1600;CS1591</NoWarn>
+    <!--
+    Note: SA1633 and SA1649 are disabled because of a bug introduced by SpecFlow 3.1.
+    If https://github.com/SpecFlowOSS/SpecFlow/pull/1790 is ever successfully merged, we should re-enable them.
+    -->
+    <NoWarn>RCS1029;RCS1089;SA1600;CS1591;SA1633;SA1649</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="System.Memory" Version="4.5.3" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.16.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
-    <PackageReference Include="SpecFlow.NUnit.Runners" Version="3.0.225" />
-    <PackageReference Include="SpecFlow.Tools.MsBuild.Generation" Version="3.0.225" />
+    <PackageReference Include="SpecFlow.NUnit.Runners" Version="3.1.78" />
+    <PackageReference Include="SpecFlow.Tools.MsBuild.Generation" Version="3.1.78" />
   </ItemGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\Ais.Net\Ais.Net.csproj" />
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="coverlet.msbuild" Version="2.7.0">
+    <PackageReference Include="coverlet.msbuild" Version="2.8.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>