"Common good" vs "System" parachain clean up (#1406)
## Summary The term "common good parachain" has been abandoned in favor of "system parachain" - e.g. [Joe's speech at Decoded2023](https://youtu.be/CSO-ERHK2gY?t=456). This pull request tries to fix and align code with this vision. ## Impact The important change is implementation of `trait IsSystem` for `Id` [here](https://github.com/paritytech/polkadot-sdk/pull/1406/files#diff-0b7b4f5b962a18ce980354592b55ab2a27b5a2e9f6f8089ec803ca73853e8583R225-R229) where we changed condition from `< 1000` to `<= 1999`, which means that all parachain IDs bellow 1999 (included) are considered as "system parachain" IDs. This change has a direct impact on the following components: #### [ChildSystemParachainAsSuperuser](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/xcm-builder/src/origin_conversion.rs#L72-L88) This origin converter is used for allowing to process XCM `Transact` from "system parachain" on the relay chain - e.g. see [configuration for Kusama](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/runtime/kusama/src/xcm_config.rs#L92-L101). Only configured for Kusama, Westend, Rococo runtimes. **No need for this feature anymore.** See [comment](https://github.com/paritytech/polkadot-sdk/pull/1406#issuecomment-1708218715). #### [IsChildSystemParachain](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/xcm-builder/src/barriers.rs#L310-L317) `IsChildSystemParachain` is used with `AllowExplicitUnpaidExecutionFrom` barrier for checking XCM programs (they have to start with `UnpaidExecution` instruction). Only configured for Kusama, Westend, Rococo runtimes. **Overall the impact is low or mostly ok because it only allows unpaid execution for "system parachains" (e.g. AssetHub, BridgeHub...) on the relay chain.** #### [SiblingSystemParachainAsSuperuser](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/xcm-builder/src/origin_conversion.rs#L94-L114) Not used anywhere in `polkadot-sdk` repo. ## Unresolved Questions - [ ] constants `LOWEST_USER_ID` and `LOWEST_PUBLIC_ID` seem to express the same thing now, do we want to keep them both or deprecated one of them? If so, which one? - [x] determine impact for `ChildSystemParachainAsSuperuser` ## TODO - [ ] when merged here, open PR to the `polkadot-fellows` ## Related Material https://youtu.be/CSO-ERHK2gY?t=456 https://forum.polkadot.network/t/polkadot-protocol-and-common-good-parachains/866 https://wiki.polkadot.network/docs/learn-system-chains