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/Cargo.toml

@@ -19,7 +19,7 @@ dlopen_derive = "0.1.4"
 dlopen = "0.1.8"
 ed25519-dalek = "1.0.1"
 fs_extra = "1.2.0"
-futures = "0.3.8"
+futures = "0.3.15"
 futures-util = "0.3.5"
 itertools = "0.9.0"
 lazy_static = "1.4.0"