SPM: use a symbolic platform constant

Author: futuretap

Package.swift

@@ -6,7 +6,7 @@ import PackageDescription
 let package = Package(
     name: "FTLinearActivityIndicator",
     platforms: [
-        .iOS("12.0")
+		.iOS(.v12)
     ],
     products: [
         // Products define the executables and libraries produced by a package, and make them visible to other packages.