Modernize SwiftPM manifest for Xcode 26 and the current Rx ecosystem

[lm2343635]

Summary

This PR refreshes the Swift Package Manager manifest as the first step of the RxController 2.0 modernization.

Changes

• bump swift-tools-version from 5.0 to 5.9
• set the next-major iOS baseline to iOS 15
• raise RxSwift's SwiftPM floor to 6.10.2
• raise RxFlow's SwiftPM floor to 2.13.0
• use explicit SwiftPM product dependencies
• keep Swift 5 language mode so current Swift 6 compilers can build the library without forcing a Swift 6 language-mode migration

Why

RxSwift 6.10.x contains explicit Xcode 26 / Swift 6.x compatibility work. Xcode 26.x supports iOS 15+ deployment targets, so the deployment-target change is intentionally scoped to the next major RxController release.

Validation

• swift package resolve
• Xcode 16.4 build
• Xcode 26.6 build

Related: #14, #15