migrations: prevent accidentally using unversioned migrations instead of VersionedMigration (#3835)

closes #1324 

#### Problem
Currently, it is possible to accidentally use inner unversioned
migration instead of `VersionedMigration` since both implement
`OnRuntimeUpgrade`.

#### Solution

With this change, we make it clear that value of `Inner` is not intended
to be used directly. It is achieved by bounding `Inner` to new trait
`UncheckedOnRuntimeUpgrade`, which has the same interface (except
`unchecked_` prefix) as `OnRuntimeUpgrade`.

#### `try-runtime` functions

Since developers can implement `try-runtime` for `Inner` value in
`VersionedMigration` and have custom logic for it, I added the same
`try-runtime` functions to `UncheckedOnRuntimeUpgrade`. I looked for a
ways to not duplicate functions, but couldn't find anything that doesn't
significantly change the codebase. So I would appreciate If you have any
suggestions to improve this

cc @liamaharon @xlc 

polkadot address: 16FqwPZ8GRC5U5D4Fu7W33nA55ZXzXGWHwmbnE1eT6pxuqcT

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/society/src/migrations.rs

@@ -19,7 +19,7 @@
 
 use super::*;
 use codec::{Decode, Encode};
-use frame_support::traits::{Defensive, DefensiveOption, Instance, OnRuntimeUpgrade};
+use frame_support::traits::{Defensive, DefensiveOption, Instance, UncheckedOnRuntimeUpgrade};
 
 #[cfg(feature = "try-runtime")]
 use sp_runtime::TryRuntimeError;
@@ -36,7 +36,7 @@ impl<
 		T: Config<I>,
 		I: Instance + 'static,
 		PastPayouts: Get<Vec<(<T as frame_system::Config>::AccountId, BalanceOf<T, I>)>>,
-	> OnRuntimeUpgrade for VersionUncheckedMigrateToV2<T, I, PastPayouts>
+	> UncheckedOnRuntimeUpgrade for VersionUncheckedMigrateToV2<T, I, PastPayouts>
 {
 	#[cfg(feature = "try-runtime")]
 	fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {