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/eframe/src/native/app_icon.rs

@@ -225,9 +225,7 @@ fn set_title_and_icon_mac(title: &str, icon_data: Option<&IconData>) -> AppIconS
     }
 
     unsafe {
-        let app = if let Some(app) = NSApp {
-            app
-        } else {
+        let Some(app) = NSApp else {
             log::debug!("NSApp is null");
             return AppIconStatus::NotSetIgnored;
         };