fix: require iOS 15 for SwiftPM

[hatemragab]

Summary

• raise the SwiftPM package deployment target to iOS 15 so the generated FlutterFramework dependency resolves
• retain CocoaPods metadata at iOS 12 for legacy Flutter toolchains, document the split, and align every example target to iOS 15
• add metadata regression tests plus an iOS CI matrix that builds and verifies this plugin through SwiftPM and CocoaPods

Compatibility

• SwiftPM: minimum iOS version changes from 12 to 15
• CocoaPods: minimum metadata remains iOS 12
• no Dart API, method-channel schema, permissions, or native runtime behavior changes

Validation

• flutter analyze
• flutter test — 96 tests passed
• cd example && flutter analyze
• cd example && flutter test — 3 tests passed
• clean flutter build ios --debug --no-codesign with SwiftPM enabled
• clean flutter build ios --debug --no-codesign with SwiftPM disabled (CocoaPods)
• generated SwiftPM and CocoaPods manifests confirmed v_video_compressor used the intended integration path
• workflow YAML parsed locally; actionlint was not installed

Closes #22