Subxt integration tests failed against latest Substrate build.

[github-actions]

The nightly CI run which downloads the latest version of Substrate ran into test failures, which likely means that there are breaking changes that need fixing in Subxt.

Go to https://github.com/paritytech/subxt/actions/workflows/nightly.yml to see details about the failure.

[lexnv]

Tests are failing because chainHead has been stabilized

thread 'full_client::client::unstable_rpcs::chainhead_unstable_body' panicked at testing/integration-tests/src/full_client/client/unstable_rpcs.rs:69:65:
called `Result::unwrap()` on an `Err` value: Rpc(ClientError(Call(ErrorObject { code: MethodNotFound, message: "Method not found", data: None })))