Merge bitcoin/bitcoin#32096: Move some tests and documentation from testnet3 to testnet4

aa7a898c23 doc: use testnet4 in developer docs (Sjors Provoost)
6c217d22fd test: use testnet4 in argsman test (Sjors Provoost)
7c200ece80 test: use testnet4 in key_io_valid.json (Sjors Provoost)
d424bd5941 test: drop unused testnet3 magic bytes (Sjors Provoost)
8cfc09fafe test: cover testnet4 magic in assumeutxo.py (Sjors Provoost)
4281e3603a zmq: use testnet4 in zmq_sub.py example (Sjors Provoost)

Pull request description:

  In preparation for dropping testnet3 entirely in #31974 this PR migrates a few things to testnet4:

  * the ZMQ examples
  * developer docs
  * various unit tests
  * the snapshot magic byte check in `feature_assumeutxo.py`

  It drops `testnet3` from `MAGIC_BYTES` in the test framework, since no test uses it.

ACKs for top commit:
  fjahr:
    re-ACK aa7a898c23
  maflcko:
    lgtm ACK aa7a898c23 🔊
  hodlinator:
    re-ACK aa7a898c23

Tree-SHA512: 235f74273234e8fb2aedf0017dea5c16bb9813ec7a1f89a51abe85691f09830a5ead834115d7db0936e12e55a40bc81888856a8002fe507c1474407e77f8b9fb

test/functional/feature_assumeutxo.py

@@ -23,10 +23,11 @@ from test_framework.compressor import (
 from test_framework.messages import (
     CBlockHeader,
     from_hex,
-    msg_headers,
-    tx_from_hex,
-    ser_varint,
+    MAGIC_BYTES,
     MAX_MONEY,
+    msg_headers,
+    ser_varint,
+    tx_from_hex,
 )
 from test_framework.p2p import (
     P2PInterface,
@@ -102,15 +103,15 @@ class AssumeutxoTest(BitcoinTestFramework):
         self.log.info("  - snapshot file with mismatching network magic")
         invalid_magics = [
             # magic, name, real
-            [0xf9beb4d9, "main", True],
-            [0x0b110907, "test", True],
-            [0x0a03cf40, "signet", True],
-            [0x00000000, "", False],
-            [0xffffffff, "", False],
+            [MAGIC_BYTES["mainnet"], "main", True],
+            [MAGIC_BYTES["testnet4"], "testnet4", True],
+            [MAGIC_BYTES["signet"], "signet", True],
+            [0x00000000.to_bytes(4, 'big'), "", False],
+            [0xffffffff.to_bytes(4, 'big'), "", False],
         ]
         for [magic, name, real] in invalid_magics:
             with open(bad_snapshot_path, 'wb') as f:
-                f.write(valid_snapshot_contents[:7] + magic.to_bytes(4, 'big') + valid_snapshot_contents[11:])
+                f.write(valid_snapshot_contents[:7] + magic + valid_snapshot_contents[11:])
             if real:
                 assert_raises_rpc_error(parsing_error_code, f"Unable to parse metadata: The network of the snapshot ({name}) does not match the network of this node (regtest).", node.loadtxoutset, bad_snapshot_path)
             else:

test/functional/test_framework/messages.py

@@ -78,10 +78,10 @@ MAX_OP_RETURN_RELAY = 83
 DEFAULT_MEMPOOL_EXPIRY_HOURS = 336  # hours
 
 MAGIC_BYTES = {
-    "mainnet": b"\xf9\xbe\xb4\xd9",   # mainnet
-    "testnet3": b"\x0b\x11\x09\x07",  # testnet3
-    "regtest": b"\xfa\xbf\xb5\xda",   # regtest
-    "signet": b"\x0a\x03\xcf\x40",    # signet
+    "mainnet": b"\xf9\xbe\xb4\xd9",
+    "testnet4": b"\x1c\x16\x3f\x28",
+    "regtest": b"\xfa\xbf\xb5\xda",
+    "signet": b"\x0a\x03\xcf\x40",
 }
 
 def sha256(s):