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/xcm/pallet-xcm/Cargo.toml

@@ -32,8 +32,6 @@ xcm-builder = { package = "staging-xcm-builder", path = "../xcm-builder" }
 
 [features]
 default = [ "std" ]
-# Enable `VersionedMigration` for the migrations using the experimental feature.
-experimental = [ "frame-support/experimental" ]
 std = [
 	"bounded-collections/std",
 	"codec/std",