Use substrate storage version

[gregdhill]

Most of the pallets currently declare StorageVersion in types.rs but we can switch to the interface defined by Substrate, for example:

/// The current storage version.
const STORAGE_VERSION: StorageVersion = StorageVersion::new(3);

#[pallet::pallet]
#[pallet::generate_store(pub(super) trait Store)]
#[pallet::storage_version(STORAGE_VERSION)]
#[pallet::without_storage_info]
pub struct Pallet<T>(_);