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>

cmd/tailscale/cli/up.go

@@ -334,8 +334,7 @@ func prefsFromUpArgs(upArgs upArgsT, warnf logger.Logf, st *ipnstate.Status, goo
 		if expr, useAutoExitNode := ipn.ParseAutoExitNodeString(upArgs.exitNodeIP); useAutoExitNode {
 			prefs.AutoExitNode = expr
 		} else if err := prefs.SetExitNodeIP(upArgs.exitNodeIP, st); err != nil {
-			var e ipn.ExitNodeLocalIPError
-			if errors.As(err, &e) {
+			if _, ok := errors.AsType[ipn.ExitNodeLocalIPError](err); ok {
 				return nil, fmt.Errorf("%w; did you mean --advertise-exit-node?", err)
 			}
 			return nil, err
@@ -912,7 +911,7 @@ func addPrefFlagMapping(flagName string, prefNames ...string) {
 	prefType := reflect.TypeFor[ipn.Prefs]()
 	for _, pref := range prefNames {
 		t := prefType
-		for _, name := range strings.Split(pref, ".") {
+		for name := range strings.SplitSeq(pref, ".") {
 			// Crash at runtime if there's a typo in the prefName.
 			f, ok := t.FieldByName(name)
 			if !ok {