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/manager_linux.go

@@ -380,7 +380,7 @@ func isLibnssResolveUsed(env newOSConfigEnv) error {
 	if err != nil {
 		return fmt.Errorf("reading /etc/resolv.conf: %w", err)
 	}
-	for _, line := range strings.Split(string(bs), "\n") {
+	for line := range strings.SplitSeq(string(bs), "\n") {
 		fields := strings.Fields(line)
 		if len(fields) < 2 || fields[0] != "hosts:" {
 			continue