Fixed
- Derive all internal contexts from user-provided `Client` context. This includes the job fetch context, notifier unlisten, and completer. [PR 514](https://github.com/riverqueue/river/pull/514).
- Lowered the `go` directives in `go.mod` to Go 1.21, which River aims to support. A more modern version of Go is specified with the `toolchain` directive. This should provide more flexibility on the minimum required Go version for programs importing River. [PR 522](https://github.com/riverqueue/river/pull/522).