Json SourceGen tests are not running on .NET Framework

[eerhardt]

These tests are using an undefined $(NetFrameworkCurrent) property. This means the tests aren't running on .NET Framework because this property is blank:

runtime/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/System.Text.Json.SourceGeneration.Unit.Tests.targets

Line 3 in 634dfa1

<TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>

runtime/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Tests.targets

Line 3 in 634dfa1

<TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>

We should also fix this one place in the docs that is using this incorrect property:

runtime/docs/coding-guidelines/project-guidelines.md

Lines 52 to 57 in 331823f

	All supported targets with unique windows/unix build for netcoreapp:
	```
	<PropertyGroup>
	<TargetFrameworks>$(NetCoreAppCurrent)-windows;$(NetCoreAppCurrent)-Unix;$(NetFrameworkCurrent)</TargetFrameworks>
	<PropertyGroup>
	```

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json
See info in area-owners.md if you want to be subscribed.

Issue Details

---

These tests are using an undefined $(NetFrameworkCurrent) property. This means the tests aren't running on .NET Framework because this property is blank:

runtime/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/System.Text.Json.SourceGeneration.Unit.Tests.targets

Line 3 in 634dfa1

<TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>

runtime/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/System.Text.Json.SourceGeneration.Tests.targets

Line 3 in 634dfa1

<TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>

We should also fix this one place in the docs that is using this incorrect property:

runtime/docs/coding-guidelines/project-guidelines.md

Lines 52 to 57 in 331823f

	All supported targets with unique windows/unix build for netcoreapp:
	```
	<PropertyGroup>
	<TargetFrameworks>$(NetCoreAppCurrent)-windows;$(NetCoreAppCurrent)-Unix;$(NetFrameworkCurrent)</TargetFrameworks>
	<PropertyGroup>
	```

Author:	eerhardt
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[jkotas]

The change was reverted in #64108