Remove nightly cfg from impl crate

We can use `rustversion` for this. For some of the tests its still
needed because of this error:

```
error[E0658]: non-inline modules in proc macro input are unstable
  --> tests/error/mod.rs:53:1
   |
53 | mod nightly;
   | ^^^^^^^^^^^^
   |
   = note: see issue #54727 <rust-lang/rust#54727> for more information
```

impl/Cargo.toml

@@ -44,7 +44,7 @@ features = ["full"]
 rustdoc-args = ["--cfg", "docsrs"]
 
 [lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ["cfg(ci)", "cfg(nightly)"] }
+unexpected_cfgs = { level = "warn", check-cfg = ["cfg(ci)"] }
 
 [features]
 default = []

impl/doc/error.md

@@ -57,11 +57,11 @@ Backtraces don't work though, because the `Backtrace` type is only available in
 ## Example usage
 
 ```rust
-# #![cfg_attr(nightly, feature(error_generic_member_access))]
+# #![rustversion::attr(nightly, feature(error_generic_member_access))]
 // Nightly requires enabling this feature:
 // #![feature(error_generic_member_access)]
-# #[cfg(not(nightly))] fn main() {}
-# #[cfg(nightly)] fn main() {
+# #[rustversion::not(nightly)] fn main() {}
+# #[rustversion::nightly] fn main() {
 # use core::error::{request_ref, request_value, Error as __};
 # use std::backtrace::Backtrace;
 #

tests/generics.rs

@@ -38,7 +38,6 @@ struct WrappedDouble<T: Clone, U: Clone>(T, U);
 #[from(forward)]
 struct WrappedDouble2<T: Clone, U: Clone>(T, U);
 
-#[cfg(nightly)]
 #[derive(
     From,
     FromStr,