Drop support for legacy target target framework

[cuteant]

Drop support for legacy target target framework

[cuteant]

Windows desktop applications(winforms/wpf) isn't fully supported in .NET Core, starting with the .NET 5.0 release, it’s not a problem anymore.
so there is really no need to keep support for .netfx
😃
'dotnetty-common/Handlers' module : still need to continue to support .net core 3.1

[maksimkim]

Is this because of alpn on sslstream?

[cuteant]

Yeah

[maksimkim]

But isn't it available in netstandard2.1?

[cuteant]

@maksimkim

I was wrong，:stuck_out_tongue: only 'dotnetty-common' module : still need to continue to support .net core 3.1

https://github.com/cuteant/SpanNetty/blob/main/src/DotNetty.Common/Internal/TextEncodings.Utf8.NetCore3.cs
https://github.com/cuteant/SpanNetty/blob/main/src/DotNetty.Common/Internal/SpanHelpers.Byte.cs

[cuteant]

what I thought was to provide a unified impl(for encoding/span) that could be called by dotnetty.buffer in every targetframework......
so I copied some code from corefx

[cuteant]

and also , like this code:

SpanNetty/src/DotNetty.Buffers/PooledUnsafeDirectByteBuffer.cs

Lines 113 to 115 in 46f4abe

	#if NETCOREAPP || NETSTANDARD_2_0_GREATER
	output.Write(_GetReadableSpan(index, length));
	#else

is able to execute in netcoreapp2.1 but not in .net standard 2.0