Clean up STJ references Fixes #39902

[Forgind]

Fixes #39902

First commit removes references to S.T.J that shouldn't be necessary because we have our own "WorkloadManifestReader.SystemTextJson" class.

The second commit forces all our package versions to 8.0.0.

I was a bit confused with this logic:
<UseSystemTextJson Condition="'$(TargetFramework)'!='netstandard2.0' And '$(TargetFramework)'!='net472'">True</UseSystemTextJson>

This seems to say that we should use System.Text.Json on core only

[kasperk81]

duplicate condition (previous line covered it)

[Forgind]

I'm just confused why things aren't working the way I'd expect, so I was adding some duplication in case I had missed something, and it didn't work as I'd thought.

[marcpopMSFT]

This property is used in directory.packages.props so what you're doing here is actually overriding the version used by transitive pinning (so overriding the version for all implicit .net core references). You'll need a separate property to use for pinning to 8.0.0 and let the d.p.props version float to current. Runtime does this too if you see here: https://github.com/dotnet/runtime/blob/main/eng/Versions.props#L137

FYI, I talked to Eric and I'll be prepping a change for 8.0.1xx and 8.0.3xx for how we handle STJ. We'll want that change in 8.0.4xx as well. It depends on a change in runtime so you won't be able to get this PR working in 4xx as it turns out. You should be able to retarget to main, make the above change, apply my changes, and get it sorted out. I'll talk to you offline as it's complicated as there are actually 3 different classes of STJ references, not 2 like I thought.