Replace deprecated prefixLength

Author: nicolasstucki

tastydoc/src/dotty/tastydoc/comment/WikiParser.scala

@@ -364,7 +364,7 @@ private[comment] final class WikiParser(
 
     // maxSkip - size of the longest common whitespace prefix of non-empty lines
     val nonEmptyLines = lines.filter(_.trim.nonEmpty)
-    val maxSkip = if (nonEmptyLines.isEmpty) 0 else nonEmptyLines.map(line => line.prefixLength(_ == ' ')).min
+    val maxSkip = if (nonEmptyLines.isEmpty) 0 else nonEmptyLines.map(line => line.iterator.takeWhile(_ == ' ').size).min
 
     // remove common whitespace prefix
     lines.map(line => if (line.trim.nonEmpty) line.substring(maxSkip) else line).mkString("\n")