bitcoin: Change rust cfg lint to warn

We are trying to release `bitcoin-io` from this branch but currently
the build for `bitcoin` fails because of buggy feature gating on then
`units 0.1.x` branch.

```rust
        #[cfg(all(feature = "alloc", not(feature = "std")))]
        $crate::impl_tryfrom_str_from_int_infallible!(&str, $to, $inner, $fn; alloc::string::String, $to, $inner, $fn; alloc::boxed::Box<str>, $to, $inner, $fn);
        #[cfg(feature = "std")]
        $crate::impl_tryfrom_str_from_int_infallible!(&str, $to, $inner, $fn; std::string::String, $to, $inner, $fn; std::boxed::Box<str>, $to, $inner, $fn);
```

FTR this no longer exists on master. From above we see that in
`bitcoin` the macro will never be called for `--no-default-feature`
builds.

This doesn't matter on this branch so just change the lint to `warn`
instead of `deny`.

Author: tcharding

bitcoin/Cargo.toml

@@ -81,4 +81,4 @@ required-features = ["std", "rand-std", "bitcoinconsensus"]
 name = "sighash"
 
 [lints.rust]
-unexpected_cfgs = { level = "deny", check-cfg = ['cfg(bench)', 'cfg(fuzzing)', 'cfg(kani)', 'cfg(mutate)', 'cfg(rust_v_1_60)'] }
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bench)', 'cfg(fuzzing)', 'cfg(kani)', 'cfg(mutate)', 'cfg(rust_v_1_60)'] }