Reset deprecation period for setPubRootDirectories (#139592)

This deprecation period is being renewed after we found it affected many IDE plugins that had not yet migrated, but did not receive a warning during the deprecation period.
We also expected to have a migration guide since the migration is not always to simply rename to addPubRootDirectories. This will give another 1+ year for the migration to be completed.

Part of https://github.com/flutter/flutter/issues/139243
Will update announcement at flutter-announce to reflect this API will remain for a while longer.

packages/flutter/lib/src/widgets/service_extensions.dart

@@ -216,7 +216,7 @@ enum WidgetInspectorServiceExtensions {
   ///   extension is registered.
   @Deprecated(
     'Use addPubRootDirectories instead. '
-    'This feature was deprecated after v3.1.0-9.0.pre.',
+    'This feature was deprecated after v3.18.0-2.0.pre.',
   )
   setPubRootDirectories,
 

packages/flutter/lib/src/widgets/widget_inspector.dart

@@ -1486,7 +1486,7 @@ mixin WidgetInspectorService {
   @protected
   @Deprecated(
     'Use addPubRootDirectories instead. '
-    'This feature was deprecated after v3.1.0-9.0.pre.',
+    'This feature was deprecated after v3.18.0-2.0.pre.',
   )
   void setPubRootDirectories(List<String> pubRootDirectories) {
     addPubRootDirectories(pubRootDirectories);