all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -1484,7 +1484,7 @@ func (c *Direct) answerPing(pr *tailcfg.PingRequest) {
 		}
 		return
 	}
-	for _, t := range strings.Split(pr.Types, ",") {
+	for t := range strings.SplitSeq(pr.Types, ",") {
 		switch pt := tailcfg.PingType(t); pt {
 		case tailcfg.PingTSMP, tailcfg.PingDisco, tailcfg.PingICMP, tailcfg.PingPeerAPI:
 			go doPingerPing(c.logf, httpc, pr, c.pinger, pt)