tailcfg: no-op bump of MapRequest.Version

So 1.18 and 1.20 don't have the same.

Change-Id: Ib2cac7c11eb37d9a0c2fcb66630f1cae619a97f4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -48,7 +48,8 @@ import (
 //    23: 2021-08-25: DNSConfig.Routes values may be empty (for ExtraRecords support in 1.14.1+)
 //    24: 2021-09-18: MapResponse.Health from control to node; node shows in "tailscale status"
 //    25: 2021-11-01: MapResponse.Debug.Exit
-const CurrentMapRequestVersion = 25
+//    26: 2022-01-12: (nothing, just bumping for 1.20.0)
+const CurrentMapRequestVersion = 26
 
 type StableID string