Update objc2 to v0.6, and use new objc2-core-graphics crate

[madsmtm]

objc2 now provides an interface to CoreGraphics, which has automatic memory management, and interfaces better with the other objc2-* crates. Note that this doesn't have, but I'd argue that's actually a benefit, since it forces us to implement the data provider ourselves (and makes it very visible that that's a place where there's room for improvement).

Another notable change is the introduction of Retained::downcast, which we use to ensure that our types are correct in the key-value observer (instead of blindly casting, and hoping that they are).

This requires an MSRV bump to Rust 1.71 to get access to extern "C-unwind" functions.