all: use Go 1.18's strings.Cut

More remain.

Change-Id: I6ec562cc1f687600758deae1c9d7dbd0d04004cb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/netmap/netmap.go

@@ -78,8 +78,8 @@ type NetworkMap struct {
 // It will neither start nor end with a period.
 func (nm *NetworkMap) MagicDNSSuffix() string {
 	name := strings.Trim(nm.Name, ".")
-	if i := strings.Index(name, "."); i != -1 {
-		name = name[i+1:]
+	if _, rest, ok := strings.Cut(name, "."); ok {
+		return rest
 	}
 	return name
 }