Update CHANGELOG.md with Kani 0.43.0 changes

Signed-off-by: Felipe R. Monteiro <[email protected]>

Author: feliperodri

CHANGELOG.md

@@ -4,6 +4,13 @@ This file contains notable changes (e.g. breaking changes, major changes, etc.)
 
 This file was introduced starting Kani 0.23.0, so it only contains changes from version 0.23.0 onwards.
 
+## [0.43.0]
+
+## What's Changed
+* Rust toolchain upgraded to `nightly-2023-12-14` by @tautschnig and @adpaco-aws
+
+**Full Changelog**: https://github.com/model-checking/kani/compare/kani-0.42.0...kani-0.43.0
+
 ## [0.42.0]
 
 ### What's Changed