lib/lwip: Update lwIP to v2.1.3, tag STABLE-2_1_3_RELEASE.

There don't seem to be many changes going from v2.1.2 to v2.1.3 of lwIP.
Mostly they are:
- IPv6 fixes and improvements
- changes to apps and other code that MicroPython doesn't use
- comments and tests
- minor bug fixes

In particular there doesn't look to be any change to the API of any
function used by MicroPython.

Network multi tests pass on PYBD_SF2 and PYBD_SF6.  PYBD_SF2, PYBD_SF6 and
PICO_W have unchanged iperf3 performance.  Similar results for networking
on the mimxrt port.

Signed-off-by: Damien George <damien@micropython.org>

lib/lwip

@@ -1 +1 @@
-Subproject commit 159e31b689577dbf69cf0683bbaffbd71fa5ee10
+Subproject commit 6ca936f6b588cee702c638eee75c2436e6cf75de