title: Removed `pallet::getter` usage from `pallet-im-online`

doc:
  - audience: Runtime Dev
    description: |
      This PR removes `pallet::getter` usage from `pallet-im-online`, and updates dependant code accordingly.
      The syntax `StorageItem::<T, I>::get()` should be used instead.

crates: 
  - name: pallet-im-online