Drop net6.0 target and use net6.0 to run netstandard2.1 tests, #1020

[paulirwin]

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Drop net6.0 target and use net6.0 to run netstandard2.1 tests

Fixes #1020

Description

This drops the out-of-support net6.0 target from the project.

Additionally, it drops the net5.0 target from the tests, which was previously used to test netstandard2.1. With the net6.0 target dropped, we now use net6.0 to test netstandard2.1. This will help alleviate issues running .NET 5 tests on macOS Arm64 as well.

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[paulirwin]

ADO build failed; will investigate. Marking as draft for now.

[paulirwin]

Build succeeds on ADO now except for the known #1034 issue.

[NightOwl888]

Thanks for the PR.

There are a few changes requested, mostly about keeping the documentation changes updated for 4.8.0-beta00017. See the comments inline.