Bump SPM watchOS deployment target to 5.0 (#2732)

Xcode 27 / Swift 6.4 PackageDescription deprecates .watchOS(.v4), so
every consumer resolving RxSwift via SPM sees:

  'v4' is deprecated: watchOS 9.0 is the oldest supported version

Despite the message, .watchOS(.v5) is not deprecated and compiles
without warnings, so this is the smallest bump that silences it.

Only the Package@swift-5.9.swift manifest is affected; the legacy
Package.swift is not picked by toolchains that emit this warning.

Fixes #2715

Author: oronbz

Package@swift-5.9.swift

@@ -96,7 +96,7 @@ extension Target {
 
 let package = Package(
     name: "RxSwift",
-    platforms: [.iOS(.v12), .macOS(.v10_13), .watchOS(.v4), .tvOS(.v12), .visionOS(.v1)],
+    platforms: [.iOS(.v12), .macOS(.v10_13), .watchOS(.v5), .tvOS(.v12), .visionOS(.v1)],
     products: ([
         [
             .library(name: "RxSwift", targets: ["RxSwift"]),