Replace Newtonsoft with System.Text.Json for generating RID graph

[kasperk81]

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[kasperk81]

@akoeplinger @jkoritzinsky

[kasperk81]

artifacts/bin/Microsoft.NETCore.Platforms/PortableRuntimeIdentifierGraph.json before and after are identical, but the checked in version of the file has inconsistent formatting

runtime/src/libraries/Microsoft.NETCore.Platforms/src/PortableRuntimeIdentifierGraph.json

Lines 1 to 5 in 57f3e7e

	{
	"runtimes": {
	"android": {
	"#import": [
	"linux-bionic"

"runtime" using 4 spaces and the rest, 2 (existing issue)

[akoeplinger]

that whitespace difference should (hopefully) not matter so I'm fine if we fix it