Update to Rust 1.76 (#4411)

Motivation: I want to replace `cargo-cranky` with workspace lints, first
available in Rust 1.74.
However, `cargo doc` would hange on `wgpu` and `wgpu-core` on 1.74 and
1.75… so now we're on 1.76.
I think this is fine - when 1.78 is released next week we're still two
versions behind the bleeding edge.

…and the branch name is just wrong 🤦

crates/epaint/src/shape.rs

@@ -1293,12 +1293,7 @@ impl std::fmt::Debug for PaintCallback {
 
 impl std::cmp::PartialEq for PaintCallback {
     fn eq(&self, other: &Self) -> bool {
-        // As I understand it, the problem this clippy is trying to protect against
-        // can only happen if we do dynamic casts back and forth on the pointers, and we don't do that.
-        #[allow(clippy::vtable_address_comparisons)]
-        {
-            self.rect.eq(&other.rect) && Arc::ptr_eq(&self.callback, &other.callback)
-        }
+        self.rect.eq(&other.rect) && Arc::ptr_eq(&self.callback, &other.callback)
     }
 }