build(deps): bump futures from 0.3.8 to 0.3.9 (solana-labs#1052)

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.3.8 to 0.3.9.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.8...0.3.9)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>