Etherlink: take into account mixHash to prevRandao renaming

What

Rename mixHash to prevRandao following changes from !13617 (merged).

Related to #7255

Why

!13617 (merged) handles EIP-4399 mixHash renaming. Tezt must be updated accordingly.

Things to consider

For backward compatibility JSON decoding looks for a mixHash field first, and, if none are found, tries prevRandao.

Manually testing the MR

tezt --file etherlink/tezt/tests/evm_rollup.ml evm rpc get_block_by_hash

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR