encoding/json: use builtin min function in appendString

To make code a bit simpler.

Change-Id: Ic5f4a2357a757ad8fe391793a0eccb067a063e26

src/encoding/json/encode.go

@@ -1015,10 +1015,7 @@ func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool)
 		// For now, cast only a small portion of byte slices to a string
 		// so that it can be stack allocated. This slows down []byte slightly
 		// due to the extra copy, but keeps string performance roughly the same.
-		n := len(src) - i
-		if n > utf8.UTFMax {
-			n = utf8.UTFMax
-		}
+		n := min(len(src)-i, utf8.UTFMax)
 		c, size := utf8.DecodeRuneInString(string(src[i : i+n]))
 		if c == utf8.RuneError && size == 1 {
 			dst = append(dst, src[start:i]...)