net: raise V1_PREFIX_LEN from 12 to 16

A "version" message in the V1 protocol starts with a fixed 16 bytes:
* The 4-byte network magic
* The 12-byte zero-padded command "version" plus 5 0x00 bytes

The current code detects incoming V1 connections by just looking at the
first 12 bytes (matching an earlier version of BIP324), but 16 bytes is
more precise. This isn't an observable difference right now, as a 12 byte
prefix ought to be negligible already, but it may become observable with
future extensions to the protocol, so make the code match the
specification.

src/net.cpp

@@ -1098,10 +1098,10 @@ void V2Transport::ProcessReceivedMaybeV1Bytes() noexcept
     AssertLockNotHeld(m_send_mutex);
     Assume(m_recv_state == RecvState::KEY_MAYBE_V1);
     // We still have to determine if this is a v1 or v2 connection. The bytes being received could
-    // be the beginning of either a v1 packet (network magic + "version\x00"), or of a v2 public
+    // be the beginning of either a v1 packet (network magic + "version\x00\x00\x00\x00\x00"), or
-    // key. BIP324 specifies that a mismatch with this 12-byte string should trigger sending of the
+    // of a v2 public key. BIP324 specifies that a mismatch with this 16-byte string should trigger
-    // key.
+    // sending of the key.
-    std::array<uint8_t, V1_PREFIX_LEN> v1_prefix = {0, 0, 0, 0, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0};
+    std::array<uint8_t, V1_PREFIX_LEN> v1_prefix = {0, 0, 0, 0, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0, 0, 0, 0, 0};
     std::copy(std::begin(Params().MessageStart()), std::end(Params().MessageStart()), v1_prefix.begin());
     Assume(m_recv_buffer.size() <= v1_prefix.size());
     if (!std::equal(m_recv_buffer.begin(), m_recv_buffer.end(), v1_prefix.begin())) {
@@ -1295,7 +1295,7 @@ size_t V2Transport::GetMaxBytesToProcess() noexcept
         // receive buffer.
         Assume(m_recv_buffer.size() <= V1_PREFIX_LEN);
         // As long as we're not sure if this is a v1 or v2 connection, don't receive more than what
-        // is strictly necessary to distinguish the two (12 bytes). If we permitted more than
+        // is strictly necessary to distinguish the two (16 bytes). If we permitted more than
         // the v1 header size (24 bytes), we may not be able to feed the already-received bytes
         // back into the m_v1_fallback V1 transport.
         return V1_PREFIX_LEN - m_recv_buffer.size();

src/net.h

@@ -473,7 +473,7 @@ private:
 
     /** The length of the V1 prefix to match bytes initially received by responders with to
      *  determine if their peer is speaking V1 or V2. */
-    static constexpr size_t V1_PREFIX_LEN = 12;
+    static constexpr size_t V1_PREFIX_LEN = 16;
 
     // The sender side and receiver side of V2Transport are state machines that are transitioned
     // through, based on what has been received. The receive state corresponds to the contents of,