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>

docs/webhooks/example.go

@@ -87,7 +87,7 @@ func verifyWebhookSignature(req *http.Request, secret string) (events []event, e
 		return nil, err
 	}
 	mac := hmac.New(sha256.New, []byte(secret))
-	mac.Write([]byte(fmt.Sprint(timestamp.Unix())))
+	mac.Write(fmt.Append(nil, timestamp.Unix()))
 	mac.Write([]byte("."))
 	mac.Write(b)
 	want := hex.EncodeToString(mac.Sum(nil))
@@ -120,8 +120,8 @@ func parseSignatureHeader(header string) (timestamp time.Time, signatures map[st
 	}
 
 	signatures = make(map[string][]string)
-	pairs := strings.Split(header, ",")
-	for _, pair := range pairs {
+	pairs := strings.SplitSeq(header, ",")
+	for pair := range pairs {
 		parts := strings.Split(pair, "=")
 		if len(parts) != 2 {
 			return time.Time{}, nil, errNotSigned