mining: only pad with OP_0 at heights <= 16

Drop the include_dummy_extranonce branch from the OP_0 padding
condition in CreateNewBlock(), so that the dummy extraNonce is
only appended when consensus actually requires it (heights <= 16,
where the BIP34 height push alone would yield a 1-byte scriptSig
and trigger bad-cb-length).

The include_dummy_extranonce option struct field is now unused by
the miner and is removed in the next commit. Callers still set it,
so that this commit compiles.

Regenerate the hardcoded coinbase / block hashes throughout the
unit and functional test suites and update the regtest assumeutxo
snapshot in chainparams.

Additional side-effects:

- Without the dummy extranonce, coinbase scriptSigs are 1 byte
  shorter at heights > 16, making every block 1 byte smaller.
  This shifts where block files wrap and therefore where pruning
  boundaries land.

- feature_assumeutxo malleation cases:
  - case 1: error message changes due to UTXO reordering, similar
            to 8f2078af6a
  - case 4: the corruption byte is swapped from \x82 to \x83
            because \x82 happened to be the actual value at that
            offset in the new snapshot.

src/kernel/chainparams.cpp

@@ -607,9 +607,9 @@ public:
         m_assumeutxo_data = {
             {   // For use by unit tests
                 .height = 110,
-                .hash_serialized = AssumeutxoHash{uint256{"b952555c8ab81fec46f3d4253b7af256d766ceb39fb7752b9d18cdf4a0141327"}},
+                .hash_serialized = AssumeutxoHash{uint256{"86e9a1205b418b16dde3a18a78c730e30137e28466bda5dbf6b33ab8fc05447c"}},
                 .m_chain_tx_count = 111,
-                .blockhash = uint256{"6affe030b7965ab538f820a56ef56c8149b7dc1d1c144af57113be080db7c397"},
+                .blockhash = uint256{"135eec25a6fb277884e5824e7aa7d052c4868161c99a5122170b5266f86c273d"},
             },
             {
                 // For use by fuzz target src/test/fuzz/utxo_snapshot.cpp
@@ -621,9 +621,9 @@ public:
             {
                 // For use by test/functional/feature_assumeutxo.py and test/functional/tool_bitcoin_chainstate.py
                 .height = 299,
-                .hash_serialized = AssumeutxoHash{uint256{"d2b051ff5e8eef46520350776f4100dd710a63447a8e01d917e92e79751a63e2"}},
+                .hash_serialized = AssumeutxoHash{uint256{"106b2c56233e378a824cf0d5ff2be42ed32c72f1605c9be288d00942908a40ac"}},
                 .m_chain_tx_count = 334,
-                .blockhash = uint256{"7cc695046fec709f8c9394b6f928f81e81fd3ac20977bb68760fa1faa7916ea2"},
+                .blockhash = uint256{"0c552ced4721c249a389eb9b08cb8da261cd46f0e7b5f9d064d48f3113406853"},
             },
         };