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>

util/httphdr/httphdr.go

@@ -44,7 +44,7 @@ func ParseRange(hdr string) (ranges []Range, ok bool) {
 	hdr = strings.Trim(hdr, ows) // per RFC 7230, section 3.2
 	units, elems, hasUnits := strings.Cut(hdr, "=")
 	elems = strings.TrimLeft(elems, ","+ows)
-	for _, elem := range strings.Split(elems, ",") {
+	for elem := range strings.SplitSeq(elems, ",") {
 		elem = strings.Trim(elem, ows) // per RFC 7230, section 7
 		switch {
 		case strings.HasPrefix(elem, "-"): // i.e., "-" suffix-length