Follow up to trimming changes for 6.x

Proposed Changes

The allows the 6.x library to be used in trimmed and native AOT'd applications without any warnings.

Since the 6.x branch doesn't target net6.0+, it only targets netstandard2.0 and net462, the #if NET6_0_OR_GREATER checks don't do anything.

To resolve this issue, and copy the trimming attributes into this library following the recommendation at https://devblogs.microsoft.com/dotnet/creating-aot-compatible-libraries/#approach-2-define-the-attributes-internally. This allows the library to apply the attributes without targeting net6.0+.

Also moving DebugUtil to the test project - porting #1009 from the main branch.

Contributes to #1410

Types of Changes

What types of changes does your code introduce to this project? Put an x in the boxes that apply

[x] Bug fix (non-breaking change which fixes issue #NNNN)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause an observable behavior change in existing systems)
[ ] Documentation improvements (corrections, new content, etc)
[ ] Cosmetic change (whitespace, formatting, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask on the mailing list. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

[x] I have read the CONTRIBUTING.md document
[x] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
[x] All tests pass locally with my changes
[x] I have added tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation (if appropriate)
[x] Any dependent changes have been merged and published in related repositories

cc @lukebakken

rabbitmq / rabbitmq-dotnet-client