Replace cargo machete with cargo shear (#8094)

We've had good experiences with `cargo shear` at Rerun

crates/egui_demo_lib/Cargo.toml

@@ -20,6 +20,9 @@ workspace = true
 all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
 
+[package.metadata.cargo-shear]
+ignored = ["image"] # We need the png feature
+
 [lib]