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/support/src/traits/hooks.rs

@@ -227,6 +227,30 @@ pub trait OnRuntimeUpgrade {
 	}
 }
 
+/// This trait is intended for use within `VersionedMigration` to execute storage migrations without
+/// automatic version checks. Implementations should ensure migration logic is safe and idempotent.
+pub trait UncheckedOnRuntimeUpgrade {
+	/// Called within `VersionedMigration` to execute the actual migration. It is also
+	/// expected that no version checks are performed within this function.
+	///
+	/// See also [`Hooks::on_runtime_upgrade`].
+	fn on_runtime_upgrade() -> Weight {
+		Weight::zero()
+	}
+
+	/// See [`Hooks::pre_upgrade`].
+	#[cfg(feature = "try-runtime")]
+	fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
+		Ok(Vec::new())
+	}
+
+	/// See [`Hooks::post_upgrade`].
+	#[cfg(feature = "try-runtime")]
+	fn post_upgrade(_state: Vec<u8>) -> Result<(), TryRuntimeError> {
+		Ok(())
+	}
+}
+
 #[cfg_attr(all(not(feature = "tuples-96"), not(feature = "tuples-128")), impl_for_tuples(64))]
 #[cfg_attr(all(feature = "tuples-96", not(feature = "tuples-128")), impl_for_tuples(96))]
 #[cfg_attr(feature = "tuples-128", impl_for_tuples(128))]
@@ -459,7 +483,9 @@ pub trait Hooks<BlockNumber> {
 	/// ## Implementation Note: Standalone Migrations
 	///
 	/// Additional migrations can be created by directly implementing [`OnRuntimeUpgrade`] on
-	/// structs and passing them to `Executive`.
+	/// structs and passing them to `Executive`. Or alternatively, by implementing
+	/// [`UncheckedOnRuntimeUpgrade`], passing it to [`crate::migrations::VersionedMigration`],
+	/// which already implements [`OnRuntimeUpgrade`].
 	///
 	/// ## Implementation Note: Pallet Versioning
 	///