Rename mixHash to prevRandao

## Issue
[EIP-4399](https://eips.ethereum.org/EIPS/eip-4399) replaces `mixHash` block header value with beacon chain RANDAO mix to expose it inside of the EVM, this EIP suggests that the field should be further renamed to `prevRandao`, thus, there is a desire to do the renaming in JSON RPC API schema.

This change would involve:
* Renaming itself
* Tests update
* EL clients update to conform the new schema

The other problem which doesn't seem to have a perfect solution is serving old PoW blocks with updated schema. One of potential solutions would be to make `mixHash` and `prevRandao` optional in the schema and filled by clients according to the context. The downside of this solution is additional engineering and testing complexity.