Deprecate RewardDestination::Controller (#2380)

Deprecates `RewardDestination::Controller` variant.

- [x] `RewardDestination::Controller` annotated with `#[deprecated]`.
- [x] `Controller` variant is now handled the same way as `Stash` in
`payout_stakers`.
- [x] `set_payee` errors if `RewardDestination::Controller` is provided.
- [x] Added `update_payee` call to lazily migrate
`RewardDestination::Controller` `Payee` storage entries to
`RewardDestination::Account(controller)` .
- [x] `payout_stakers_dead_controller` has been removed from benches &
weights - was not used.
- [x] Tests no longer use `RewardDestination::Controller`.

---------

Co-authored-by: command-bot <>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: georgepisaltu <52418509+georgepisaltu@users.noreply.github.com>

substrate/frame/staking/src/lib.rs

@@ -240,9 +240,9 @@
 //! [`Payee`] storage item (see
 //! [`set_payee`](Call::set_payee)), to be one of the following:
 //!
-//! - Controller account, (obviously) not increasing the staked value.
 //! - Stash account, not increasing the staked value.
 //! - Stash account, also increasing the staked value.
+//! - Any other account, sent as free balance.
 //!
 //! ### Additional Fund Management Operations
 //!
@@ -404,7 +404,9 @@ pub enum RewardDestination<AccountId> {
 	Staked,
 	/// Pay into the stash account, not increasing the amount at stake.
 	Stash,
-	/// Pay into the controller account.
+	#[deprecated(
+		note = "`Controller` will be removed after January 2024. Use `Account(controller)` instead. This variant now behaves the same as `Stash` variant."
+	)]
 	Controller,
 	/// Pay into a specified account.
 	Account(AccountId),