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/identity/src/migration.rs

@@ -16,7 +16,9 @@
 //! Storage migrations for the Identity pallet.
 
 use super::*;
-use frame_support::{migrations::VersionedMigration, pallet_prelude::*, traits::OnRuntimeUpgrade};
+use frame_support::{
+	migrations::VersionedMigration, pallet_prelude::*, traits::UncheckedOnRuntimeUpgrade,
+};
 
 #[cfg(feature = "try-runtime")]
 use codec::{Decode, Encode};
@@ -66,7 +68,7 @@ pub mod v1 {
 	/// prevent stalling a parachain by accumulating too much weight in the migration. To have an
 	/// unlimited migration (e.g. in a chain without PoV limits), set this to `u64::MAX`.
 	pub struct VersionUncheckedMigrateV0ToV1<T, const KL: u64>(PhantomData<T>);
-	impl<T: Config, const KL: u64> OnRuntimeUpgrade for VersionUncheckedMigrateV0ToV1<T, KL> {
+	impl<T: Config, const KL: u64> UncheckedOnRuntimeUpgrade for VersionUncheckedMigrateV0ToV1<T, KL> {
 		#[cfg(feature = "try-runtime")]
 		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
 			let identities = v0::IdentityOf::<T>::iter().count();