This PR removes all references to the elliptic functions that were deprecated in golang 1.22.
Instead this PR does the following:

We change S256() to return the raw bitcurve instead of the elliptic.Curve, this has two effects:
- Every instance of S256() can call the unmarshall and marshall operations
- The package is not neatly abstracted away anymore, which we can debate if we want to change that

We also modify the behavior in the ecies (and rlpx) package a bit.
Previously all curves were acceptable, now only curves that implement the marshaller and unmarshaller
interface are allowed. All common curves have this, so its not an issue in our code.
Dependencies maybe need to implement the (un-)marshaller interfaces though.