Apply existential any to protocol for Swift 6

• https://github.com/swiftlang/swift-evolution/blob/main/proposals/0335-existential-any.md

  After any is introduced, warnings will be added to guide programmers toward the new syntax. Finally, these warnings can become errors, or plain protocol names can be repurposed, in Swift 6.

• I refactored by apply .enableUpcomingFeature("ExistentialAny")

  let package = Package(
      name: "Kingfisher",
      platforms: [.iOS(.v13), .macOS(.v10_15), .tvOS(.v13), .watchOS(.v6)],
      products: [
          .library(name: "Kingfisher", targets: ["Kingfisher"])
      ],
      targets: [
          .target(
              name: "Kingfisher",
              path: "Sources",
              swiftSettings: [
                  .enableUpcomingFeature("ExistentialAny")
              ]
          )
      ]
  )