#1479 Rework conditional packages in v6 branch including adding net 6

[thompson-tomo]

Proposed Changes

With this change the polyfill package references have a condition placed on them so that they are only added for the net standard 2.0 & net 462 TFM and not the newly added net 6.0 TFM.

Types of Changes

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• New feature (non-breaking change which adds functionality) Add net 6.0 so that conditional packages can be defined #1479

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Further Comments

Closes: #1479

[lukebakken]

@thompson-tomo by introducing the net6.0 target, this PR no longer builds, due to conditional code here:

https://github.com/thompson-tomo/rabbitmq-dotnet-client/blob/enhancement/%231479_ConditionalPackagesV6/projects/RabbitMQ.Client/util/NetworkOrderDeserializer.cs

However, if I change

#elif NETSTANDARD

to just #else, then the test suite fails with framing errors.

https://github.com/rabbitmq/rabbitmq-dotnet-client/actions/runs/7893177389/job/21541242245

If you run RabbitMQ locally, and the test suite, you will see RabbitMQ log errors such as these:

[error] <0.750.0> closing AMQP connection <0.750.0> (10.0.0.2:35296 -> 172.17.0.2:5672):
[error] <0.750.0> {handshake_error,starting,0,
[error] <0.750.0>                  {amqp_error,frame_error,
[error] <0.750.0>                              "type 1, all octets = <<>>: {invalid_frame_end_marker,0}",
[error] <0.750.0>                              none}}

Could you please investigate?

[lukebakken]

@danielmarbach when you made this comment, I'm assuming the kinds of failures you were predicting are exactly what I see exposed in this PR (comment)