Remove getters from pallet-membership (paritytech#4840)

As per paritytech#3326 , removes pallet::getter macro usage from
pallet-membership. The syntax StorageItem::<T, I>::get() should be used
instead. Also converts some syntax to turbo and reimplements the removed
getters, following paritytech#223

cc @muraca

---------

Co-authored-by: Dónal Murray <donalm@seadanda.dev>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

prdoc/pr_4840.prdoc

@@ -0,0 +1,14 @@
+# 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 pallet-membership
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR removed the `pallet::getter`s from `pallet-membership`.
+      The syntax `StorageItem::<T, I>::get()` should be used instead.
+
+crates:
+  - name: pallet-membership
+    bump: minor