revert deletion of rewrite notice

Author: bishabosha

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3350,6 +3350,7 @@ final class DeprecatedAssignmentSyntax(key: Name, value: untpd.Tree)(using Conte
       |not as an assignment.
       |
       |To assign a value, use curly braces: `{${key} = ${value}}`."""
+      + Message.rewriteNotice("This", version = SourceVersion.`3.6-migration`)
 
   override protected def explain(using Context): String = ""
 

Diff for: tests/warn/21681.check

@@ -5,3 +5,4 @@
   |  not as an assignment.
   |
   |  To assign a value, use curly braces: `{age = 29}`.
+  |  This can be rewritten automatically under -rewrite -source 3.6-migration.

Diff for: tests/warn/21681b.check

@@ -5,3 +5,4 @@
   |  not as an assignment.
   |
   |  To assign a value, use curly braces: `{age = 29}`.
+  |  This can be rewritten automatically under -rewrite -source 3.6-migration.

Diff for: tests/warn/21681c.check

@@ -5,3 +5,4 @@
   |  not as an assignment.
   |
   |  To assign a value, use curly braces: `{age = 29}`.
+  |  This can be rewritten automatically under -rewrite -source 3.6-migration.

Diff for: tests/warn/21770.check

@@ -5,3 +5,4 @@
   |         not as an assignment.
   |
   |         To assign a value, use curly braces: `{cache = Some(i)}`.
+  |         This can be rewritten automatically under -rewrite -source 3.6-migration.