Note pg15 support in changelog

Signed-off-by: Smittyvb <[email protected]>

Author: syvb

Changelog.md

@@ -14,6 +14,7 @@ This changelog should be updated as part of a PR if the work is worth noting (mo
   We can at least prevent folks from running the candlestick aggregates in parallel mode and hitting this bug by dropping support for partial aggregation until we've resolved the issue.
 
 #### Other notable changes
+- [#646](https://github.com/timescale/timescaledb-toolkit/pull/646): Added support for PostgreSQL 15.
 
 #### Shout-outs