use a more modern writing style to simplify code (#7182)

Signed-off-by: joemicky <joemickychang@outlook.com>
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/server.go

@@ -985,10 +985,10 @@ func trustedRealClientIP(r *http.Request, headers []string, clientIP string) str
 
 	// Since there can be many header values, we need to
 	// join them together before splitting to get the full list
-	allValues := strings.Split(strings.Join(values, ","), ",")
+	allValues := strings.SplitSeq(strings.Join(values, ","), ",")
 
 	// Get first valid left-most IP address
-	for _, part := range allValues {
+	for part := range allValues {
 		// Some proxies may retain the port number, so split if possible
 		host, _, err := net.SplitHostPort(part)
 		if err != nil {