Update minimum macOS version to 15 in Package.swift

Raised the minimum supported macOS version from 14 to 15 in the Swift package manifest to reflect updated platform requirements.

Author: rismay

Package.swift

@@ -27,7 +27,7 @@ var commonShellBenchDependencies: [Target.Dependency] = [
 let package = Package(
   name: "CommonShell",
   platforms: [
-    .macOS(.v14), .iOS(.v17), .macCatalyst(.v17),
+    .macOS(.v15), .iOS(.v17), .macCatalyst(.v17),
   ],
   products: [
     .library(name: "CommonShell", targets: ["CommonShell"]),