Merge #15891: test: Require standard txs in regtest by default

fa89badf88 test: Require standard txs in regtest (MarcoFalke)
fa9b419160 test: Add test that mainnet requires standard txs (MarcoFalke)
fa613ca0a8 chainparams: Remove unused fMineBlocksOnDemand (MarcoFalke)

Pull request description:

  I don't see a reason why regtest should allow non-standard txs, as it makes testing mainnet behaviour such as #15846 unnecessarily hard and unintuitive.

  Of course, testnet policy remains unchanged to allow propagation of non-standard txs.

ACKs for top commit:
  ajtowns:
    ACK fa89badf88

Tree-SHA512: c4c675affb054868850bd2683aa07f4c741a448cbacb2ea8334191e105f426b0790fe6a468be61e9c5880d24154f7bf1c7075051697172dce92180c1bc3a1c90

doc/release-notes.md

@@ -103,6 +103,20 @@ Low-level Changes section below.
 Low-level changes
 =================
 
+RPC
+---
+
+
+Tests
+-----
+
+- The regression test chain, that can be enabled by the `-regtest` command line
+  flag, now requires transactions to not violate standard policy by default.
+  Making the default the same as for mainnet, makes it easier to test mainnet
+  behavior on regtest. Be reminded that the testnet still allows non-standard
+  txs by default and that the policy can be locally adjusted with the
+  `-acceptnonstdtxn` command line flag for both test chains.
+
 Configuration
 ------------