fix: strip unnecessary path separator from migration backup path

taipy/core/_entity/_migrate/_migrate_fs.py

@@ -98,7 +98,7 @@ def _migrate_fs_entities(path: str, backup: bool = True) -> bool:
         return False
 
     if backup:
-        backup_path = f"{path}_backup"
+        backup_path = path.rstrip("\\/") + "_backup"
         try:
             shutil.copytree(path, backup_path)
         except FileExistsError: