Remove NUnit.Agent.Core from this repo and use a reference

[CharliePoole]

No description provided.

[manfred-brands]

Instead of having to repeat the version, even with the msbuild properties, you could use central package management and only specify the version in Directory.packages.props
See https://learn.microsoft.com/en-us/nuget/consume-packages/central-package-management

See also remark on .slnx

Both can be done separately from this PR.

[manfred-brands]

Could be much simpler if migrating to .slnx file.
There is a dotnet sln migrate command for it.

[CharliePoole]

@manfred-brands
Thanks for the review. Both of those items are on my radar but, as you say, seem better done separately. I'll add issues for these.

NUnit.Engine.Api and NUnit.Agent.Core use .slnx and I only had to upgrade NuGet to make it work.

Experimenting with central package management a few years ago, I ran into problems with multiple package sources (NU1507 warning). However I see that Terje has made it work without having to use package source mapping, so we should give that a try.