Fix UWP compatibility after System.Text.Json update

[sb123sb123]

Fixes #967

Issue #967 reports that v6.6.1 and v6.6.2 fail the UWP .NET Native Toolchain with "Sequence contains more than one element", while v6.6.0 works. The dependency group update reintroduced System.Text.Json 10.x for netstandard2.0 consumers. This pins System.Text.Json to 9.0.2, the version used by the previously accepted UWP fix. JsonExtensions is pinned to 1.2.0 because its netstandard2.0 dependency in 1.2.3 requires System.Text.Json 10.0.8 or newer; the older package remains compatible with the 9.0.2 baseline. CS8002 is scoped to netstandard2.0 packaging because the internalized legacy package is unsigned.

Tests:

• dotnet build YoutubeExplode/YoutubeExplode.csproj -c Release -f netstandard2.0 --no-restore — passed with 0 warnings and 0 errors.
• dotnet test YoutubeExplode.Tests/YoutubeExplode.Tests.csproj -c Release -f net48 --no-restore --filter FullyQualifiedName~LegacyTargetFrameworkSpecs — passed: 1 test.
• The regression test asserts that a net48 consumer resolves System.Text.Json major version 9.

Limitations:

• This host has no installed .NET workloads, UWP project, or .NET Native toolchain, so Store packaging and native compilation were not run. The narrow regression test verifies the package graph and assembly version; it does not prove every UWP project configuration.

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