dev: using official version of discord.net

docs: upped version to 5.1.5
dev: removed nuget.config as we no longer rely on myget
dev: Fixed some build warnings

src/NadekoBot/NadekoBot.csproj

@@ -4,7 +4,7 @@
         <Nullable>enable</Nullable>
         <ImplicitUsings>true</ImplicitUsings>
         <SatelliteResourceLanguages>en</SatelliteResourceLanguages>
-        <Version>5.1.4</Version>
+        <Version>5.1.5</Version>
 
         <!-- Output/build -->
         <RunWorkingDirectory>$(MSBuildProjectDirectory)</RunWorkingDirectory>
@@ -29,7 +29,7 @@
         </PackageReference>
         <PackageReference Include="CodeHollow.FeedReader" Version="1.2.6"/>
         <PackageReference Include="CommandLineParser" Version="2.9.1"/>
-        <PackageReference Include="Discord.Net" Version="3.204.0"/>
+        <PackageReference Include="Discord.Net" Version="3.15.3"/>
         <PackageReference Include="CoreCLR-NCalc" Version="3.1.246"/>
         <PackageReference Include="Google.Apis.Urlshortener.v1" Version="1.41.1.138"/>
         <PackageReference Include="Google.Apis.YouTube.v3" Version="1.68.0.3414"/>