tailcfg: bump CurrentMapRequestVersion, forgotten earlier

In f45a9e291b (2021-03-04), I tried to bump CurrentMapRequestVersion
to 12 but only documented the meaning of 12 but forgot to actually
increase it from 11.

Mapver 11 was added in ea49b1e811 (2021-03-03).

Fix this in its own commit so we can cherry-pick it to the 1.6 release
branch.

tailcfg/tailcfg.go

@@ -36,7 +36,7 @@ import (
 //    10: 2021-01-17: client understands MapResponse.PeerSeenChange
 //    11: 2021-03-03: client understands IPv6, multiple default routes, and goroutine dumping
 //    12: 2021-03-04: client understands PingRequest
-const CurrentMapRequestVersion = 11
+const CurrentMapRequestVersion = 12
 
 type StableID string