Update default compilation to C23 (from C11)

[ppannuto]

This is basically a no-op at this point, as C23 didn't really break anything*.

The prior selection of C11 was mostly arbitrary. At the time the first
libtock-c build infrastructure was built, C17 hadn't been ratified yet,
but we needed some newer stuff than C99 provided, so we explicitly set
things to C11 (well, gnu11).

We have an impending need for the new C23 #embed compiler directive,
so this version bump lays the groundwork.

Future PRs may also look at other new C23 features; there are many that
are likely useful to libtock-c.

*There was one function signature it's more persnickety about, which
is the other commit in this PR.

[ppannuto]

Actually hold on, there's a few more (hopefully equally inconsequential) things to fix, e.g.

[-bash] Wed 07 May 16:33 [[bump-to-c23] ~/temp/libtock-c/examples/tests/hail]
$ make
/Applications/Xcode.app/Contents/Developer/usr/bin/make -C ../../../libnrfserialization -f Makefile.setup all
make[1]: Nothing to be done for `all'.
/Applications/Xcode.app/Contents/Developer/usr/bin/make -C ../../../libnrfserialization -f Makefile all
  CC        ../libnrfserialization/nrf5x-base-f813b5dbfef3b7a71044bca72e2ab8b3c4d9e593/sdk/nrf51_sdk_11.0.0/components/libraries/util/app_error.c
../libnrfserialization/nrf5x-base-f813b5dbfef3b7a71044bca72e2ab8b3c4d9e593/sdk/nrf51_sdk_11.0.0/components/libraries/util/app_error.c: In function 'app_error_save_and_stop':
../libnrfserialization/nrf5x-base-f813b5dbfef3b7a71044bca72e2ab8b3c4d9e593/sdk/nrf51_sdk_11.0.0/components/libraries/util/app_error.c:118:5: error: implicit declaration of function '__disable_irq' [-Wimplicit-function-declaration]
  118 |     __disable_irq();
      |     ^~~~~~~~~~~~~

Turns out changing Configuration.mk doesn't force a re-build [and no, I'm not fixing that].

[alevy]

I don't have a strong opinion about this. In principle, since this is mostly a no-op, it just makes it hard for people with older compilers to build this, on the other hand... let's use the new thing!