bevy_app: Apply #[deny(clippy::allow_attributes, clippy::allow_attributes_without_reason)] (#17095)

# Objective
We want to deny the following lints:
* `clippy::allow_attributes` - Because there's no reason to
`#[allow(...)]` an attribute if it wouldn't lint against anything; you
should always use `#[expect(...)]`
* `clippy::allow_attributes_without_reason` - Because documenting the
reason for allowing/expecting a lint is always good

## Solution
Set the `clippy::allow_attributes` and
`clippy::allow_attributes_without_reason` lints to `deny`, and bring
`bevy_reflect` in line with the new restrictions.

No code changes have been made - except if a lint that was previously
`allow(...)`'d could be removed via small code changes. For example,
`unused_variables` can be handled by adding a `_` to the beginning of a
field's name.

## Testing
I ran `cargo clippy`, and received no errors.

crates/bevy_app/src/lib.rs

@@ -7,6 +7,7 @@
 )]
 #![cfg_attr(any(docsrs, docsrs_dep), feature(doc_auto_cfg, rustdoc_internals))]
 #![forbid(unsafe_code)]
+#![deny(clippy::allow_attributes, clippy::allow_attributes_without_reason)]
 #![doc(
     html_logo_url = "https://bevyengine.org/assets/icon.png",
     html_favicon_url = "https://bevyengine.org/assets/icon.png"

crates/bevy_app/src/plugin.rs

@@ -183,7 +183,10 @@ mod sealed {
             where
                 $($plugins: Plugins<$param>),*
             {
-                // We use `allow` instead of `expect` here because the lint is not generated for all cases.
+                #[expect(
+                    clippy::allow_attributes,
+                    reason = "This is inside a macro, and as such, may not trigger in all cases."
+                )]
                 #[allow(non_snake_case, reason = "`all_tuples!()` generates non-snake-case variable names.")]
                 #[allow(unused_variables, reason = "`app` is unused when implemented for the unit type `()`.")]
                 #[track_caller]