💚Move tests to net7 to fix build

[schnerring]

• Test fixes: cherry-picked from Support [DataMember] IsRequired in NewtonsoftDataContractResolver #2644
• Add some preprocessor symbols for .NET 7 support

[jerryk414]

@schnerring Is there a timeline or any blockers on merging this in along with #2671?

Really looking forward to being able to use discriminators in .NET 7 without having to add duplicate code.

[schnerring]

Since creating the PR, I haven't looked at this again. I guess it would be good timing to sync this with the upcoming .NET 8 changes? But I haven't checked if this is tracked anywhere.

[martincostello]

Thanks for contributing - if you'd like to continue with this pull request, please rebase against the default branch to pick up our new CI.

From looking at the description only, some parts of this will have been addressed by #2775.

[martincostello]

Thanks for the contribution. The TFM updates are covered by the changes in #2799 to add support for .NET 8, and the other changes look like they were covered with #2775.