chore(changelog): add api breaking changelog entry from #1857 (#2157)

Closes: #XXX

## What is the purpose of the change

#1857 introduced an API break but a changelog entry wasn't added. This PR follows through with the entry

CHANGELOG.md

@@ -66,6 +66,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * [#1630](https://github.com/osmosis-labs/osmosis/pull/1630) Delete the v043_temp module, now that we're on an updated SDK version.
 * [#1667](https://github.com/osmosis-labs/osmosis/pull/1673) Move wasm-bindings code out of app package into its own root level package.
 * [#2013](https://github.com/osmosis-labs/osmosis/pull/2013) Make `SetParams`, `SetPool`, `SetTotalLiquidity`, and `SetDenomLiquidity` GAMM APIs private
+*[#1857](https://github.com/osmosis-labs/osmosis/pull/1857) x/mint rename GetLastHalvenEpochNum to GetLastReductionEpochNum
 
 ### Features