Remove DisableSpecificWarnings suppressing C4996 for legacy APIs

[iamAbhi-916]

Ref : #15517
We're silencing the C4996 deprecation warnings because the deprecated headers are being pulled in transitively from upstream React Native's New Architecture code (react/runtime/ReactInstance.h → jsireact/JSIExecutor.h → legacy cxxreact headers.

it's upstream React Native that still includes these legacy headers. Until React Native completes their legacy removal, we can't fix these at the source.

RN use of legacy apis in 0.82, verify at ref : https://github.com/facebook/react-native/tree/9731e8ebc5ea87526a91b9903172639e062cd920/packages/react-native/ReactCommon/cxxreact

ReactCommon.vcxproj (Upstream RN cxxreact files)

• cxxreact/ReactMarker.cpp
• cxxreact/JSExecutor.cpp
• cxxreact/Instance.cpp
• cxxreact/NativeToJsBridge.cpp
• cxxreact/JSIndexedRAMBundle.cpp
• cxxreact/MethodCall.cpp
• cxxreact/RAMBundleRegistry.cpp
• cxxreact/ModuleRegistry.cpp
• jsiexecutor/jsireact/JSIExecutor.cpp
• jsiexecutor/jsireact/JSINativeModules.cpp

##Shared.vcxitems (RNW code with transitive dependencies)

• Microsoft.ReactNative/ReactCoreInjection.cpp
• Microsoft.ReactNative/ReactHost/CrashManager.cpp
• Microsoft.ReactNative/ReactHost/MsoReactContext.cpp
• Microsoft.ReactNative/ReactHost/ReactInstanceWin.cpp
• react/renderer/core/EventQueueProcessor.cpp (upstream)
• react/renderer/uimanager/UIManager.cpp (upstream)
• react/runtime/ReactInstance.cpp (upstream)

Test Projects

• Desktop.UnitTests/InstanceMocks.cpp
• IntegrationTests/TestModule.cpp
• Desktop.IntegrationTests/Modules/TestDevSettingsModule.cpp
• Desktop.IntegrationTests/Modules/TestImageLoaderModule.cp