Replace `api::set_code_hash` with `api::set_code_hash2` impl

[ascjones]

The existing set_code_hash impl accepts the concrete &[u8; 32], which is different to other api methods handling code hashes which use the generic E::Hash type.

In order not to make a breaking change, I have proposed adding a new set_code_hash2 method to the api which accepts the generic E::Hash instead.

This ticket is a reminder to replace the set_code_hash impl with set_code_hash2 impl at the next MAJOR release.

[ascjones]

Closed by #1906