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>

net/dns/openresolv.go

@@ -82,7 +82,7 @@ func (m openresolvManager) GetBaseConfig() (OSConfig, error) {
 
 	// Remove the "tailscale" snippet from the list.
 	args := []string{"-l"}
-	for _, f := range strings.Split(strings.TrimSpace(string(bs)), " ") {
+	for f := range strings.SplitSeq(strings.TrimSpace(string(bs)), " ") {
 		if f == "tailscale" {
 			continue
 		}