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/web/storage.rs

@@ -31,7 +31,7 @@ pub(crate) fn load_memory(_: &egui::Context) {}
 
 #[cfg(feature = "persistence")]
 pub(crate) fn save_memory(ctx: &egui::Context) {
-    match ctx.memory(|mem| ron::to_string(mem)) {
+    match ctx.memory(ron::to_string) {
         Ok(ron) => {
             local_storage_set("egui_memory_ron", &ron);
         }