Rename mixHash to prevRandao

[mkalinin]

Issue

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.

[lightclient]

I definitely think proof-of-work blocks should return mixHash and not prevRandao.

I think the best thing to do is to return mixHash and prevRandao for post-merge blocks with the understanding that the use of mixHash is deprecated for post-merge blocks and tools should migrate to prevRandao. Later on down the road we can stop returning mixHash altogether for post-merge blocks.