Use Rust-1.78 #[diagnostic::on_unimplemented] to clarify `push_next…

…` trait

When a user passes an invalid type into `push_next()`, [since Rust 1.78]
we can now customize the diagnostics error to more clearly decsribe what
and why they are not allowed to pass a certain type into this structure
instead of having to refer to the trait definition (which contains no
docs either).

[since Rust 1.78]: https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html#diagnostic-attributes