Remove `pallet::getter` usage from authority-discovery pallet (#4091)

As per #3326, removes pallet::getter usage from the pallet
authority-discovery. The syntax `StorageItem::<T, I>::get()` should be
used instead.

cc @muraca

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_4091.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Removed `pallet::getter` usage from the authority-discovery pallet
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR removed `pallet::getter`s from `pallet-authority-discovery`s storage items.
+      When accessed inside the pallet, use the syntax `StorageItem::<T, I>::get()`.
+      When accessed outside the pallet, use the getters current_authorities() and next_authorities() instead.
+
+crates:
+  - name: pallet-authority-discovery
+    bump: major

substrate/frame/authority-discovery/src/lib.rs

@@ -48,13 +48,11 @@ pub mod pallet {
 	}
 
 	#[pallet::storage]
-	#[pallet::getter(fn keys)]
 	/// Keys of the current authority set.
 	pub(super) type Keys<T: Config> =
 		StorageValue<_, WeakBoundedVec<AuthorityId, T::MaxAuthorities>, ValueQuery>;
 
 	#[pallet::storage]
-	#[pallet::getter(fn next_keys)]
 	/// Keys of the next authority set.
 	pub(super) type NextKeys<T: Config> =
 		StorageValue<_, WeakBoundedVec<AuthorityId, T::MaxAuthorities>, ValueQuery>;