MacOS increase version to 10.15 (#2343)

Author: spetrescu84

Package.swift

@@ -5,7 +5,7 @@ import PackageDescription
 let package = Package(
   name: "MSAL",
   platforms: [
-        .macOS(.v10_13),.iOS(.v14)
+        .macOS(.v10_15),.iOS(.v14)
   ],
   products: [
       .library(