deps: Update nas.git to rebased branch on top of v1.2.1
We have a forked repo of ETSI's nas.git on our osmocom gitea, which contains an extra fix for upstream (currently waiting to be merged [1]) and one patch implementing C++ helpers to fit our own environment, which needs to be kept downstream.
There has been quite a lot of development in upstream lately (TTF_T059), with eg. old NGAP.git code being put inside nas.git (we are still using our own copy of NGAP inside osmo-ttcn3-hacks/library/, but that may change in the future).
A new branch rebased on top of upstream's master (v1.2.1) has been pushed to our forked repo, so use it here to already align better with upstream, easing further work and contributions in the future.
