tailcfg: bump capver to 72 to restore UPnP

Actually fixed in 77ff705545 but that was cherry-picked to a branch
and we don't bump capver in branches.

This tells the control plane that UPnP should be re-enabled going
forward.

Updates #8992

Change-Id: I5c4743eb52fdee94175668c368c0f712536dc26b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -110,7 +110,8 @@ type CapabilityVersion int
 //   - 69: 2023-08-16: removed Debug.LogHeap* + GoroutineDumpURL; added c2n /debug/logheap
 //   - 70: 2023-08-16: removed most Debug fields; added NodeAttrDisable*, NodeAttrDebug* instead
 //   - 71: 2023-08-17: added NodeAttrOneCGNATEnable, NodeAttrOneCGNATDisable
-const CurrentCapabilityVersion CapabilityVersion = 71
+//   - 72: 2023-08-23: TS-2023-006 UPnP issue fixed; UPnP can now be used again
+const CurrentCapabilityVersion CapabilityVersion = 72
 
 type StableID string