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>

polkadot/xcm/pallet-xcm/src/migration.rs

@@ -19,7 +19,7 @@ use crate::{
 };
 use frame_support::{
 	pallet_prelude::*,
-	traits::{OnRuntimeUpgrade, StorageVersion},
+	traits::{OnRuntimeUpgrade, StorageVersion, UncheckedOnRuntimeUpgrade},
 	weights::Weight,
 };
 
@@ -35,7 +35,7 @@ pub mod v1 {
 	///
 	/// Use experimental [`MigrateToV1`] instead.
 	pub struct VersionUncheckedMigrateToV1<T>(sp_std::marker::PhantomData<T>);
-	impl<T: Config> OnRuntimeUpgrade for VersionUncheckedMigrateToV1<T> {
+	impl<T: Config> UncheckedOnRuntimeUpgrade for VersionUncheckedMigrateToV1<T> {
 		fn on_runtime_upgrade() -> Weight {
 			let mut weight = T::DbWeight::get().reads(1);