In the 1.27 unstable releases we set the min-version to iOS15,
which means we have 50 MBytes of RAM in the Network Extension.
https://tailscale.com/blog/go-linker/
Include the UPnP/NAT-PMP/PCP portmapper support now that there
is memory for it.
Fixes https://github.com/tailscale/tailscale/issues/2495
Signed-off-by: Denton Gentry <dgentry@tailscale.com>