doc: update release notes for 2.18.2 (#1915)

Author: jhamman

docs/release.rst

@@ -18,26 +18,14 @@ Release notes
    See `GH1777 <https://github.com/zarr-developers/zarr-python/issues/1777>`_ for more details on the upcoming
    3.0 release.
 
-.. _unreleased:
-
-Unreleased
-----------
+.. _release_2.18.2:
 
 Enhancements
 ~~~~~~~~~~~~
 
-Docs
-~~~~
-
-Maintenance
-~~~~~~~~~~~
-
 * Add Zstd codec to old V3 code path.
   By :user:`Ryan Abernathey <rabernat>` 
 
-Deprecations
-~~~~~~~~~~~~
-
 .. _release_2.18.1:
 
 2.18.1