Starting April 2023 it's no longer possible to submit apps with Xcode
13.x. This technically doesn't drop _support_, but we might won't be
testing on the oldest supported version of Xcode 13, since it's not
necessary.
### Changes:
_See
[`UpdatingRequirements.md`](https://github.com/RevenueCat/purchases-ios/blob/f8e07f07e8e842c25755f3dbca3a3f35961258a0/Contributing/UpdatingRequirements.md)_
- Removed `CircleCI` Xcode 13.2.1 job (_this saves ~11 minutes of build
time on every commit_)
- Updated `README`
- Updated message in `SwiftVersionCheck.swift`
Note that a lot of CI tests still run on Xcode 13.x for now because
installing old simulators on Xcode 14.x was failing. We'll need to fix
this when `CircleCI` drops support for `Xcode 13.x`.
Once we do that, we can refactor a lot of code like
```swift
#if swift(>=5.7)
extension CustomerInfo: Sendable {}
#else
// `@unchecked` because:
// - `Date` is not `Sendable` until Swift 5.7
// - `URL` is not `Sendable` until Swift 5.7
extension CustomerInfo: @unchecked Sendable {}
#endif
```
This will make it easier to figure out that the version is no longer
compatible.
Aditionally, I've added `UpdatingRequirements.md` so we have a single
place to reference when changing these requirements.
NachoSoto