Fix broken doc link to `tokio_stream::Stream` (#2271)

* Fix broken doc link to `Stream`

This commit fixes broken doc link to `Stream` in codegen clients. That
target is `tokio_stream::Stream`, which in turn is a re-export of
`futures_core::Stream`. Since we do not have a good way to link to the
re-export, we remove a hyper link and just put `Stream`.

* Update CHANGELOG.next.toml

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Co-authored-by: Yuki Saito <awsaito@amazon.com>