add changelog entry

Signed-off-by: Onur Özkan <work@onurozkan.dev>

Author: onur-ozkan

protocols/gossipsub/CHANGELOG.md

@@ -1,3 +1,8 @@
+## next-release-version
+
+- Add fallible sequence number generation and error handling.
+  See [PR 6211](https://github.com/libp2p/rust-libp2p/pull/6211).
+
 ## 0.50.0
 - Prevent mesh exceeding mesh_n_high.
   See [PR 6184](https://github.com/libp2p/rust-libp2p/pull/6184)