Fix upstream Zarr compatibility (#9884)

Closes #9880

Author: dcherian

xarray/backends/zarr.py

@@ -1135,9 +1135,7 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
                 else:
                     encoded_attrs[DIMENSION_KEY] = dims
 
-                encoding["exists_ok" if _zarr_v3() else "overwrite"] = (
-                    True if self._mode == "w" else False
-                )
+                encoding["overwrite"] = True if self._mode == "w" else False
 
                 zarr_array = self._create_new_array(
                     name=name,