Update MSRV to Rust 1.70.0 (#3310)

* Update to Rust 1.70

* Fix renamed clippy lint

* Use let-else more

* Code cleanup

* Remove derelict Safety comments

* Enable more clippy lints

crates/egui_plot/src/items/mod.rs

@@ -1,4 +1,5 @@
 //! Contains items that can be added to a plot.
+#![allow(clippy::type_complexity)] // TODO(emilk): simplify some of the callback types with type aliases
 
 use std::ops::RangeInclusive;