By default, Rust targets iOS 7, which is unnecessary but fine for our
purposes. However, BoringSSL has some assembly files that were getting
compiled for the "latest" version (based on the SDK). Address both of
these by setting IPHONEOS_DEPLOYMENT_TARGET explicitly when building
as a CocoaPod.
While here, bump the CocoaPod minimum iOS version to 12.2, which
matches the main Signal app and will allow SignalCoreKit to do the
same.
- Skip building for Catalyst in pull request testing, but make up for
it in the "Slow Tests".
- Update the README now that the Arm Mac simulator is a Tier 2 Rust
target.
- Remove workaround for one-time incompatibility with the Arm Mac
simulator.
