Add StringBuilder.repeat from Java 21

https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/StringBuilder.html#repeat(int,int)

PiperOrigin-RevId: 707986992

src/com/google/javascript/jscomp/LightweightMessageFormatter.java

@@ -339,7 +339,7 @@ public String formatLine(String line, int lineNumber) {
 
         // nice spaces for the line number
         int spaces = numberLength - Integer.toString(lineNumber).length();
-        builder.append(" ".repeat(spaces));
+        builder.repeat(" ", spaces);
         builder.append(lineNumber);
         builder.append("| ");