Expose one init with @objc and remove min version

No other packages still specify `platforms`, and also make one `init`
method `@objc` so that the code can be shared with OpenVanilla.

Author: lukhnos

Packages/CandidateUI/Package.swift

@@ -5,7 +5,6 @@ import PackageDescription
 
 let package = Package(
     name: "CandidateUI",
-    platforms: [.macOS(.v10_15)],
     products: [
         .library(
             name: "CandidateUI",

Packages/CandidateUI/Sources/CandidateUI/CandidateController.swift

@@ -28,7 +28,7 @@ public class CandidateKeyLabel: NSObject {
     @objc public private(set) var key: String
     @objc public private(set) var displayedText: String
 
-    public init(key: String, displayedText: String) {
+    @objc public init(key: String, displayedText: String) {
         self.key = key
         self.displayedText = displayedText
         super.init()