[frame/im-online] remove ensure from pre_upgrade

modify ensure in post_upgrade to allow for future upgrades
refs paritytech#14251 (comment)

frame/im-online/src/migration.rs

@@ -61,8 +61,6 @@ pub mod v1 {
 	impl<T: Config> OnRuntimeUpgrade for Migration<T> {
 		#[cfg(feature = "try-runtime")]
 		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
-			ensure!(StorageVersion::get::<Pallet<T>>() == 0, "can only upgrade from version 0");
-
 			let count = v0::ReceivedHeartbeats::<T>::iter().count();
 			log::info!(target: TARGET, "Migrating {} received heartbeats", count);
 
@@ -111,7 +109,7 @@ pub mod v1 {
 					old_received_heartbeats
 				);
 			}
-			ensure!(StorageVersion::get::<Pallet<T>>() == 1, "must upgrade");
+			ensure!(StorageVersion::get::<Pallet<T>>() >= 1, "must upgrade");
 
 			Ok(())
 		}