Remove `derivative` dependency in favour of `educe`

The `derivative` dependency is now unmaintained. We use it to customise `derive`d implementations of traits.

In our dependent crates we've already started switching to `educe` which offers similar functionality. We can switch Lighthouse over to `educe` too once those deps are updated, which will happen as part of:

- https://github.com/sigp/lighthouse/pull/6915