Bump futures-executor from 0.3.8 to 0.3.21

Bumps [futures-executor](https://github.com/rust-lang/futures-rs) from 0.3.8 to 0.3.21.
- [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...0.3.21)

---
updated-dependencies:
- dependency-name: futures-executor
  dependency-type: indirect
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.lock

@@ -1864,9 +1864,9 @@ checksum = "0c09fd04b7e4073ac7156a9539b57a484a8ea920f79c7c675d05d289ab6110d3"
 
 [[package]]
 name = "futures-executor"
-version = "0.3.8"
+version = "0.3.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4caa2b2b68b880003057c1dd49f1ed937e38f22fcf6c212188a121f08cf40a65"
+checksum = "9420b90cfa29e327d0429f19be13e7ddb68fa1cccb09d65e5706b8c7a749b8a6"
 dependencies = [
  "futures-core",
  "futures-task",