Update CHANGELOG.md with Kani 0.43.0 changes

Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>

CHANGELOG.md

@@ -7,14 +7,7 @@ This file was introduced starting Kani 0.23.0, so it only contains changes from
 ## [0.43.0]
 
 ## What's Changed
-* Make `codegen_span` stable by @ouz-a in https://github.com/model-checking/kani/pull/2882
-* Change reachability module to use StableMIR by @celinval in https://github.com/model-checking/kani/pull/2894
-* Fix StableMIR migration documentation by @celinval in https://github.com/model-checking/kani/pull/2915
-* Stabilize `find_debug_info` by @ouz-a in https://github.com/model-checking/kani/pull/2912
-* Migrate hooks and place modules to use mostly StableMIR APIs by @celinval in https://github.com/model-checking/kani/pull/2910
-* Migrate static handling and most of the operand codegen code to StableMIR by @celinval in https://github.com/model-checking/kani/pull/2931
-* Migrate `rvalue` and `coercion` module to use StableMIR by @celinval in https://github.com/model-checking/kani/pull/2938
-* Migrate `intrinsics` module to use StableMIR by @celinval in https://github.com/model-checking/kani/pull/2939
+* Update the rust toolchain 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