change forbid for warnings & unused to deny

a new warning was caused by the `?` operator as it contains
`#[allow(unreachable_code)]`, which clashes with the previous `forbid`
rule.

as `forbid` does not allow `deny` to override it this would cause a
compile error, however due to a compiler bug this was previously not
reported at all and has now been changed to a warning in a recent(-ish)
version of rustc and will eventually become a compile error.

with `deny` it's still possible to use `allow`, thus there's no warning

see also rust-lang/rust#76053

thanks @jannic

Author: rursprung

src/lib.rs

@@ -14,10 +14,10 @@
 //! * `defmt`: you can enable the `defmt` feature to get a `defmt::Format` implementation for all structs & enums in this crate and a `defmt::debug` call for every speed change.
 
 #![forbid(unsafe_code)]
-#![forbid(warnings)]
+#![deny(warnings)]
 #![forbid(missing_docs)]
 #![forbid(missing_debug_implementations)]
-#![forbid(unused)]
+#![deny(unused)]
 #![no_std]
 
 #[cfg(feature = "defmt")]