Package vulnerability updates

[xantari]

Motivation

Which issue does this fix? Fixes #399

NOTES:

1. AutoFixture does not have a new release available that updates some of it's transitive dependencies with vulnerabilities so those were updated by pinning the newer version of the assembly in those projects that used that Autofixture nuget package.
2. xunit had dependencies that were marked as vulnerable so that was updated as well. This also involved changing "async void" to "async Task" as the "async void" methods were showing xunit deprecation notices.
3. CS8981 pragma warning added to remove that noise from the build output
4. Since this package now targets .net 8 and above, removed most of the older .net core 6 references to their .net 8 counterparts

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

Ran all automated tests, manual testing would probably be good as well however.