meta: Fix .NET Core 3.0 compatibility + Drop NS1.3 (#1382)

* Update all dependencies and deal with warning/errors

* Add updated AsyncEnumerable implementation

* Fix broken target

* Cleanup

* Remove obsolete message

* typo

* Update azure pipelines

* Update samples to .NET Core 3.0

* Pull out test change

* Install the .net core 3 SDK on the ubuntu image for the time being

* Target net core 3 for the unit tests because pipelines

src/Discord.Net.Providers.WS4Net/Discord.Net.Providers.WS4Net.csproj

@@ -1,10 +1,10 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <Import Project="../../Discord.Net.targets" />
   <PropertyGroup>
     <AssemblyName>Discord.Net.Providers.WS4Net</AssemblyName>
     <RootNamespace>Discord.Providers.WS4Net</RootNamespace>
     <Description>An optional WebSocket client provider for Discord.Net using WebSocket4Net</Description>
-    <TargetFramework>netstandard1.3</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />

src/Discord.Net.Providers.WS4Net/WS4NetProvider.cs

@@ -1,4 +1,4 @@
-using Discord.Net.WebSockets;
+using Discord.Net.WebSockets;
 
 namespace Discord.Net.Providers.WS4Net
 {