Remove `#[unstable]` on `call_v2`, `instantiate_v2`,
`lock_delegate_dependency` and `unlock_delegate_dependency`.
See ink! integrations: 
- call_v2: https://github.com/paritytech/ink/pull/2077
- instantiate_v2: <TODO>
- lock/unlock dependency: https://github.com/paritytech/ink/pull/2076