[api-upload-refactor] Set deployment target to iOS 15, simplify upload

- Package.swift: .iOS(.v13) -> .iOS(.v15), .watchOS(.v6) -> .v8
  (README already stated iOS 15; watchOS 8 needed for URLSession.upload(for:fromFile:))
- drop the iOS-13 uploadTask continuation bridge; both upload overloads now
  share one runUpload using session.upload(for:fromFile:delegate:)
- remove now-redundant @available(iOS 15) from the progress overload and test
- Documentation/API.md: drop iOS-version caveats

Package.swift

@@ -24,10 +24,10 @@ let targetDependencies: [Target.Dependency] = enableCryptoBinary
 let package = Package(
     name: "LCEssentials",
     platforms: [
-        .iOS(.v13),
+        .iOS(.v15),
         .macOS(.v10_15),
         .tvOS(.v13),
-        .watchOS(.v6)
+        .watchOS(.v8)
     ],
     products: [
         .library(