build: use newer source for libnatpmp

The source we are currently using is from 2015. The upstream repo has
received a small number of bug fixes and improvements since then.
Including one that fixes an issue for Windows users:
https://github.com/miniupnp/libnatpmp/pull/13.

The source we are currently using is the most recent "official" release,
however I don't think it's worth waiting for a new one. The maintainer
was prompted to do so in Oct 2020, then again in Jan of this year, and
no release has eventuated. Given libnatpmp is a new inclusion into our
repository, I think we should be using this newer source.

This also cleans up a few warnings we currently see in depends builds:
```bash
Extracting libnatpmp...
/home/ubuntu/bitcoin/depends/sources/libnatpmp-20150609.tar.gz: OK
Preprocessing libnatpmp...
Configuring libnatpmp...
Building libnatpmp...
make[1]: Entering directory '/home/ubuntu/bitcoin/depends/work/build/x86_64-w64-mingw32/libnatpmp/20150609-13efa1beb87'
x86_64-w64-mingw32-gcc -Os -fPIC -Wall -DENABLE_STRNATPMPERR   -c -o natpmp.o natpmp.c
x86_64-w64-mingw32-gcc -Os -fPIC -Wall -DENABLE_STRNATPMPERR   -c -o getgateway.o getgateway.c
natpmp.c:42: warning: "EWOULDBLOCK" redefined
   42 | #define EWOULDBLOCK WSAEWOULDBLOCK
      |
In file included from natpmp.c:38:
/usr/share/mingw-w64/include/errno.h:166: note: this is the location of the previous definition
  166 | #define EWOULDBLOCK 140
      |
natpmp.c:43: warning: "ECONNREFUSED" redefined
   43 | #define ECONNREFUSED WSAECONNREFUSED
      |
In file included from natpmp.c:38:
/usr/share/mingw-w64/include/errno.h:110: note: this is the location of the previous definition
  110 | #define ECONNREFUSED 107
      |
natpmp.c:271:5: warning: ‘readnatpmpresponseorretry’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
  271 | int readnatpmpresponseorretry(natpmp_t * p, natpmpresp_t * response)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
ar crs libnatpmp.a natpmp.o getgateway.o
make[1]: Leaving directory '/home/ubuntu/bitcoin/depends/work/build/x86_64-w64-mingw32/libnatpmp/20150609-13efa1beb87'
Staging libnatpmp...
Postprocessing libnatpmp...
Caching libnatpmp...
```

0 files changed, 0 insertions, 0 deletions