Update Doc/library/gzip.rst

set versionchanged to next

Author: morotti

Doc/library/gzip.rst

@@ -216,7 +216,7 @@ The module defines the following items:
       The *mtime* parameter now defaults to 0 for reproducible output.
       For the previous behaviour of using the current time,
       pass ``None`` to *mtime*.
-   .. versionchanged:: 3.14
+   .. versionchanged:: next
       The default compression level was reduced to 6 (down from 9).
       It is the default level used by most compression tools and a better
       tradeoff between speed and performance.