Fix more NU1510 warnings

Avoid referencing System.Text.Json for `netx.0`.

Author: martincostello

build/Common.props

@@ -33,7 +33,7 @@
     <TargetFrameworksForLibraries>net10.0;net9.0;net8.0;netstandard2.0;$(NetFrameworkMinimumSupportedVersion)</TargetFrameworksForLibraries>
     <TargetFrameworksForLibrariesExtended>net10.0;net9.0;net8.0;netstandard2.1;netstandard2.0;$(NetFrameworkMinimumSupportedVersion)</TargetFrameworksForLibrariesExtended>
     <TargetFrameworksForPrometheusAspNetCore>net10.0;net9.0;net8.0</TargetFrameworksForPrometheusAspNetCore>
-    <TargetFrameworksRequiringSystemTextJsonDirectReference>net8.0;netstandard2.1;netstandard2.0;$(NetFrameworkMinimumSupportedVersion)</TargetFrameworksRequiringSystemTextJsonDirectReference>
+    <TargetFrameworksRequiringSystemTextJsonDirectReference>netstandard2.1;netstandard2.0;$(NetFrameworkMinimumSupportedVersion)</TargetFrameworksRequiringSystemTextJsonDirectReference>
 
     <!-- non-production TFMs -->
     <TargetFrameworksForAspNetCoreTests>net10.0;net9.0;net8.0</TargetFrameworksForAspNetCoreTests>