Migrate foreign assets v3::Location to v4::Location (#4129)
In the move from XCMv3 to XCMv4, the `AssetId` for `ForeignAssets` in `asset-hub-rococo` and `asset-hub-westend` was left as `v3::Location` to be later migrated to `v4::Location`. This is that migration PR. Because the encoding of `v3::Location` and `v4::Location` is the same, we don't need to do any data migration, the keys will still be decodable. The [original idea by Jan](https://github.com/paritytech/polkadot/pull/7236) was to make the v4 changes in v3 since the ABI (the encoding/decoding) didn't change. Corroborated the ABI is the same iterating over all storage, the code is on [another branch](https://github.com/paritytech/polkadot-sdk/blob/cisco-assert-v3-v4-encodings-equal/cumulus/parachains/runtimes/assets/migrations/src/foreign_assets_to_v4/mod.rs). We will need a data migration when we want to update from `v4::Location` to `v5::Location` because of [the accepted RFC changing the NetworkId enum](https://github.com/polkadot-fellows/RFCs/pull/108). I'll configure MBMs (Multi-Block Migrations) then and make the actual migration. Fixes https://github.com/paritytech/polkadot-sdk/issues/4128 --------- Signed-off-by:Oliver Tale-Yazdi <[email protected]> Co-authored-by: