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>

tsnet/tsnet_test.go

@@ -2598,7 +2598,7 @@ func buildDNSQuery(name string, srcIP netip.Addr) []byte {
 		0x00, 0x01, // QDCOUNT: 1
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // ANCOUNT, NSCOUNT, ARCOUNT
 	}
-	for _, label := range strings.Split(name, ".") {
+	for label := range strings.SplitSeq(name, ".") {
 		dns = append(dns, byte(len(label)))
 		dns = append(dns, label...)
 	}