chore: use Union types instead of OneOf library based implementation when targeting .NET 11

[filzrev]

This PR contains following changes.

1. Add net11.0 target framework (.NET 11 RC is released with go-live license)
2. Replace OneOf library based implementation to use C# 15 union types when targeting net11.0.

Note: As commented at following lines.

docfx/src/Docfx.Build/OneOfJsonConverterFactory.cs

Lines 40 to 43 in 7fe76a7

	// NOTE:
	// This is a pretty slow implementation by attempting to deserialize into every possible union case.
	// It also depends on marking discriminator properties as required.
	foreach (var (type, cast) in s_types)

Existing OneOf based implementation is too slow (Especially when running under debugger)