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.

substrate/frame/support/test/tests/versioned_migration.rs

@@ -17,7 +17,7 @@
 
 //! Tests for [`VersionedMigration`]
 
-#![cfg(all(feature = "experimental", feature = "try-runtime"))]
+#![cfg(feature = "try-runtime")]
 
 use frame_support::{
 	construct_runtime, derive_impl,