Clean up pallet-contracts documentation (paritytech#10438)

* Update CHANGELOG.md

* Remove COMPLEXITY.md

frame/contracts/CHANGELOG.md

@@ -23,13 +23,9 @@ In other words: Upgrading this pallet will not break pre-existing contracts.
 - Allow contracts to dispatch calls into the runtime (**unstable**)
 [#9276](https://github.com/paritytech/substrate/pull/9276)
 
-- New **unstable** version of `seal_call` that offers more features.
+- New version of `seal_call` that offers more features.
 [#8909](https://github.com/paritytech/substrate/pull/8909)
 
-- New **unstable** `seal_rent_params` and `seal_rent_status` contract callable function.
-[#8231](https://github.com/paritytech/substrate/pull/8231)
-[#8780](https://github.com/paritytech/substrate/pull/8780)
-
 - New `instantiate` RPC that allows clients to dry-run contract instantiation.
 [#8451](https://github.com/paritytech/substrate/pull/8451)
 
@@ -38,6 +34,10 @@ In other words: Upgrading this pallet will not break pre-existing contracts.
 
 ### Changed
 
+- Replaced storage rent with automatic storage deposits
+[#9669](https://github.com/paritytech/substrate/pull/9669)
+[#10082](https://github.com/paritytech/substrate/pull/10082)
+
 - Replaced `seal_println` with the `seal_debug_message` API which allows outputting debug
 messages to the console and RPC clients.
 [#8773](https://github.com/paritytech/substrate/pull/8773)