Prepare for objc2 frameworks v0.3 (#5624)

The next version of the `objc2` framework crates will have a bunch of
default features enabled, see
https://github.com/madsmtm/objc2/issues/627, so this PR pre-emptively
disables them, so that your compile times down blow up once you upgrade
to the next version (which is yet to be released, but will be soon).

* [x] I have followed the instructions in the PR template

crates/eframe/Cargo.toml

@@ -177,12 +177,14 @@ wgpu = { workspace = true, optional = true, features = [
 # mac:
 [target.'cfg(any(target_os = "macos"))'.dependencies]
 objc2 = "0.5.1"
-objc2-foundation = { version = "0.2.0", features = [
+objc2-foundation = { version = "0.2.0", default-features = false, features = [
+  "std",
   "block2",
   "NSData",
   "NSString",
 ] }
-objc2-app-kit = { version = "0.2.0", features = [
+objc2-app-kit = { version = "0.2.0", default-features = false, features = [
+  "std",
   "NSApplication",
   "NSImage",
   "NSMenu",