test: set segwit height back to 0 on regtest

This was changed in #22818 from 0 to 1. Since it changes
BLOCK_OPT_WIT of the genesis block, older versions of bitcoin
core would not read regtest directories created with newer versions
without a reindex.

Github-Pull: bitcoin#24527
Rebased-From: 5ce3057

Author: mzumsande

src/chainparams.cpp

@@ -397,7 +397,7 @@ class CRegTestParams : public CChainParams {
         consensus.BIP65Height = 1;  // Always active unless overridden
         consensus.BIP66Height = 1;  // Always active unless overridden
         consensus.CSVHeight = 1;    // Always active unless overridden
-        consensus.SegwitHeight = 1; // Always active unless overridden
+        consensus.SegwitHeight = 0; // Always active unless overridden
         consensus.MinBIP9WarningHeight = 0;
         consensus.powLimit = uint256S("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         consensus.nPowTargetTimespan = 14 * 24 * 60 * 60; // two weeks