Use `core::error::Error`

As mentioned in https://github.com/bitcoindevkit/bdk/pull/2115#discussion_r2834065195:

> [`core::error::Error` was stabilized in 1.81](https://doc.rust-lang.org/nightly/core/error/trait.Error.html), as MSRV is 1.85 now, I think we can remove this (and all the others too)