revert prev_hash optionality in DeclareMiningJob

Author: GitGab19

06-Job-Declaration-Protocol.md

@@ -149,7 +149,7 @@ A request sent by JDC that proposes a selected set of transactions to JDS.
 | request_id                  | U32                   | Unique identifier for pairing the response                                                                                                                                                                                                                                                                                                                                                                                   |
 | mining_job_token            | B0_255                | Previously reserved mining job token received by AllocateMiningJobToken.Success                                                                                                                                                                                                                                                                                                                                              |
 | version                     | U32                   | Version header field. To be later modified by BIP320-consistent changes.                                                                                                                                                                                                                                                                                                                                                     |
-| prev_hash                   | U256                  | Optional. Contains the chain tip that the template is based on. JDS SHOULD accept templates based on different tips, as long as they are on the best block height and don't contain double spends.                                                                                                                                                                                                                           |
+| prev_hash                   | U256                  | Contains the chain tip that the template is based on. JDS SHOULD accept templates based on different tips, as long as they are on the best block height and don't contain double spends.                                                                                                                                                                                                                           |
 | coinbase_tx_prefix         | B0_64K                 | Serialized bytes representing the initial part of the coinbase transaction (not including extranonce)                                                                                                                                                                                                                                                                                                                                                                                      |
 | coinbase_tx_suffix         | B0_64K                 | Serialized bytes representing the final part of the coinbase transaction (after extranonce)                                                                                                                                                                                                                                                                                     |
 | tx_ids_list               | SEQ0_64K[U256] | List of hashes of the transaction set contained in the template. JDS checks the list against its mempool and requests missing txs via `ProvideMissingTransactions`. Does not include the coinbase transaction (as there is no corresponding full data for it yet).                                                                                                                                                          |