Fix a few issues in GP legacy to v3 conversion operator.

* Use name of old GP data for the new one, _not_ the object name.
* Properly decrease usercount of old GP ID.

Pull Request: https://projects.blender.org/blender/blender/pulls/118366

source/blender/editors/object/object_add.cc

@@ -3232,10 +3232,15 @@ static int object_convert_exec(bContext *C, wmOperator *op)
       }
 
       GreasePencil *new_grease_pencil = static_cast<GreasePencil *>(
-          BKE_id_new(bmain, ID_GP, newob->id.name + 2));
+          BKE_id_new(bmain, ID_GP, gpd->id.name + 2));
       newob->data = new_grease_pencil;
       newob->type = OB_GREASE_PENCIL;
 
+      /* NOTE: Could also use #BKE_id_free_us, to also free the legacy GP if not used anymore? */
+      id_us_min(&gpd->id);
+      /* No need to increase usercount of `new_grease_pencil`, since ID creation already set it
+       * to 1. */
+
       bke::greasepencil::convert::legacy_gpencil_to_grease_pencil(
           *bmain, *new_grease_pencil, *gpd);