Stabilize VersionedMigration (#1503)

`VersionedMigration` has become somewhat widely used for handling
version bumps in migrations the last few months.

It is currently behind the `experimental` feature flag, requiring every
pallet that writes a new migration with version bumps to set up the
`experimental` flag in their own Cargo.tomls, and also for every runtime
using these pallets to explicitly enable the `experimental` flag for
each pallet.

This is becoming quite verbose, and I can only see the number of pallets
requiring the experimental flag increasing for no other reason than
using what has become a commonly used feature.

Additionally, I'm writing migration docs and would like to avoid
stepping through how to use the `experimental` feature to get
`VersionedMigration` working.

Since the feature has been used in production for some time now without
any reported issues, is becoming commonly used and ready to advertise in
docs, I feel this is a good time to make it non-experimental.

polkadot/runtime/common/src/assigned_slots/migration.rs

@@ -63,7 +63,6 @@ pub mod v1 {
 	/// [`MigrateToV1`] wrapped in a
 	/// [`VersionedMigration`](frame_support::migrations::VersionedMigration), ensuring the
 	/// migration is only performed when on-chain version is 0.
-	#[cfg(feature = "experimental")]
 	pub type VersionCheckedMigrateToV1<T> = frame_support::migrations::VersionedMigration<
 		0,
 		1,

polkadot/runtime/common/src/paras_registrar/migration.rs

@@ -60,7 +60,6 @@ impl<T: Config, UnlockParaIds: Contains<ParaId>> OnRuntimeUpgrade
 	}
 }
 
-#[cfg(feature = "experimental")]
 pub type VersionCheckedMigrateToV1<T, UnlockParaIds> =
 	frame_support::migrations::VersionedMigration<
 		0,