Drop support for legacy target target framework

[maksimkim]

Seems like explicit support for net451,net471,netcore2.1,netcore3.1 complicates dependency management.
Does it make sense to completely drop support for net451 and support the rest through netstandard 2.0?
So list of supported versions will reduce to netstanard2.0, netstandard2.1, net5.0.

[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