Add changelog entry for state_agg stabilization

Author: syvb

Changelog.md

@@ -11,7 +11,11 @@ This changelog should be updated as part of a PR if the work is worth noting (mo
 #### Bug fixes
 - [#715](https://github.com/timescale/timescaledb-toolkit/pull/715): Fix out-of-bounds indexing error in `state_agg` rollup
 
+#### Stabilized features
+- [#723](https://github.com/timescale/timescaledb-toolkit/pull/723): Stabilized `state_agg`
+
 #### Other notable changes
+- [#723](https://github.com/timescale/timescaledb-toolkit/pull/723): Added `state_at` function for `state_agg`
 
 #### Shout-outs