☂️ Migrate all Flutter Objective-C code to Swift

Umbrella issue to track all of the Objective-C to Swift migration work within Flutter and 1p Flutter packages

- [x] https://github.com/flutter/flutter/issues/41129
- [x] https://github.com/flutter/flutter/issues/144791
- [x] https://github.com/flutter/flutter/issues/148586
- [x] https://github.com/flutter/flutter/issues/169683
- [ ] https://github.com/flutter/flutter/issues/23955
- [ ] https://github.com/flutter/flutter/issues/119015
- [ ] https://github.com/flutter/flutter/issues/71873
- [ ] https://github.com/flutter/flutter/issues/137368
- [ ] https://github.com/flutter/flutter/issues/158288
- [ ] https://github.com/flutter/flutter/issues/172799
- [ ] https://github.com/flutter/flutter/issues/175878

Note: developers will continue to be able to mix Swift and Objective-C in projects and packages as long as Apple (Xcode, Swift Package Manager, etc) allows it. However Flutter will no longer generate Objective-C code, it will be up to developers to handle Objective-C integration themselves, as with any native darwin project.