Bump futures from 0.3.8 to 0.3.15 (#17574)

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.3.8 to 0.3.15.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-lang/futures-rs/compare/0.3.8...0.3.15)

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

ledger-tool/Cargo.toml

@@ -14,7 +14,7 @@ bs58 = "0.4.0"
 bytecount = "0.6.0"
 clap = "2.33.1"
 csv = "1.1.3"
-futures = "0.3.8"
+futures = "0.3.15"
 futures-util = "0.3.5"
 histogram = "*"
 itertools = "0.9.0"