Align CocoaPods metadata and README with the 2.0 compatibility baseline

[lm2343635]

Summary

This PR documents the 2026 maintenance refresh and aligns the legacy CocoaPods path with the planned RxController 2.0 support baseline.

Changes

• mark the project as actively maintained again in 2026
• document the Xcode / Swift / iOS / Rx dependency compatibility target
• recommend Swift Package Manager for new integrations
• keep CocoaPods as a legacy transition path
• raise the podspec iOS deployment target to iOS 15
• update the RxFlow CocoaPods dependency floor to 2.13
• normalize the podspec to Swift 5 language mode
• update the Example lockfile to CocoaPods-published RxFlow 2.13.0

Notes

RxSwift's newer GitHub releases have moved toward SwiftPM/Carthage and deprecated CocoaPods support, so this PR intentionally does not claim RxSwift 6.10.x availability through CocoaPods.

RxFlow has newer Git tags usable by SwiftPM, but CocoaPods currently publishes 2.13.0; the legacy Example lockfile therefore intentionally uses 2.13.0 while the SwiftPM manifest accepts the 2.13.x line.

Validation

• CocoaPods dependency resolution succeeds with RxFlow 2.13.0
• Example workspace build succeeds in GitHub Actions

Related: #14, #15