build: migrate UnsafeTypeCastWarningLevel to CppCompileWarningSettings (UE 5.6)

[rsoudani]

Summary

Unreal Engine 5.6 deprecated ModuleRules.UnsafeTypeCastWarningLevel.
This PR migrates it to the new ModuleRules.CppCompileWarningSettings.UnsafeTypeCastWarningLevel API.

Motivation

Building the plugin on UE 5.6+ currently emits CS0618 deprecation warnings.
This change removes those warnings while keeping behavior identical.

Implementation

• Added version-guarded macros:
  • UE_5_6_OR_LATER → use CppCompileWarningSettings.UnsafeTypeCastWarningLevel
  • UE_5_0_OR_LATER → use legacy UnsafeTypeCastWarningLevel
• Preserves the same strictness (WarningLevel.Error).
• No runtime impact; Build.cs only.

Build & Validation

Built successfully using the documented workflow:

msbuild -p:UnrealEngine=5.6 -p:OutputPath=C:\_out\VSTUE_Build

✔️ Build succeeded with 0 errors.
ℹ️ Warnings observed:

• Visual Studio 2022 compiler not preferred (informational).
• C4996 deprecations in VisualStudioToolsCommandlet.cpp related to UClass::ClassDefaultObject
  (unrelated to this PR; can be addressed in a separate change by using GetDefault<>/GetMutableDefault<>).