Fix #10692: Update rewrite help message

Co-authored-by: Guillaume Martres <smarter@ubuntu.com>

Author: liufengyun

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -378,7 +378,7 @@ object Parsers {
 
     def rewriteNotice(additionalOption: String = "") = {
       val optionStr = if (additionalOption.isEmpty) "" else " " ++ additionalOption
-      i"\nThis construct can be rewritten automatically under$optionStr -rewrite."
+      i"\nThis construct can be rewritten automatically under$optionStr -rewrite -source 3.0-migration."
     }
 
     def syntaxVersionError(option: String, span: Span) =

tests/neg-custom-args/fatal-warnings/i9266.check

@@ -2,4 +2,4 @@
 3 |def test = { implicit x: Int => x + x } // error
   |                      ^
   |                      This syntax is no longer supported; parameter needs to be enclosed in (...)
-  |                      This construct can be rewritten automatically under -rewrite.
+  |                      This construct can be rewritten automatically under -rewrite -source 3.0-migration.

tests/neg-custom-args/newline-braces.check

@@ -4,4 +4,4 @@
   |  This opening brace will start a new statement in Scala 3.
   |  It needs to be indented to the right to keep being treated as
   |  an argument to the previous expression.
-  |  This construct can be rewritten automatically under -rewrite.
+  |  This construct can be rewritten automatically under -rewrite -source 3.0-migration.