fix: correctly merge IPHONEOS_DEPLOYMENT_TARGET in xcconfigs

[edusperoni]

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What is the current behavior?

When using xcodeproj merge, it'll merge properties like:

// file 1
IPHONEOS_DEPLOYMENT_TARGET= 10.0;
// file 2
IPHONEOS_DEPLOYMENT_TARGET= 14.0;

//output
IPHONEOS_DEPLOYMENT_TARGET= 10.0; 14.0;

This causes an issue with Xcode 14.3 where this property is now being set for plugins as well

What is the new behavior?

If both configs have IPHONEOS_DEPLOYMENT_TARGET, then we override instead of append

Fixes triniwiz/nativescript-plugins#176