(Minor) Compiler warnings on rust >=1.66 (const_err lint)

[optout21]

On Rust toolchain v >= 1.66, 3 compiler warnings are generated, due to usage of #[deny(const_err)], which has been removed, and converted to a hard error.

To fix:

• Remove these lines, relying on the fact that CI has builds with newer compiler version, so any eventual mis-use will be detected
  or
• Wrap these lines in conditional compilation (e.g. #[rustversion::before(1.66)]), so that it's active only on older versions. However, making a directive conditional may not be trivial.

warning: lint `const_err` has been removed: converted into hard error, see issue #71800 <https://github.com/rust-lang/rust/issues/71800> for more information
    --> lightning/src/ln/channelmanager.rs:1253:8
     |
1253 | #[deny(const_err)]
     |        ^^^^^^^^^
     |
     = note: `#[warn(renamed_and_removed_lints)]` on by default

[TheBlueMatt]

We've not bothered to remove it, as we wanted to keep older versions of rustc (at least from our MSRV) enforcing the const-err (which we use as assertions in various places). We definitely shouldn't add a dependency to use the rustversion conditional compilation, but I think we could probably remove them now given most folks are using toolchains where const_err generates the warning.