bevy_a11y: Apply #[deny(clippy::allow_attributes, clippy::allow_attributes_without_reason)] (#17093)

# 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_a11y` 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_a11y/src/lib.rs

@@ -1,4 +1,5 @@
 #![forbid(unsafe_code)]
+#![deny(clippy::allow_attributes, clippy::allow_attributes_without_reason)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![doc(
     html_logo_url = "https://bevyengine.org/assets/icon.png",