Bump futures-channel from 0.3.18 to 0.3.21

Bumps [futures-channel](https://github.com/rust-lang/futures-rs) from 0.3.18 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/0.3.18...0.3.21)

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

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

Cargo.lock

@@ -506,9 +506,9 @@ dependencies = [
 
 [[package]]
 name = "futures-channel"
-version = "0.3.18"
+version = "0.3.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7fc8cd39e3dbf865f7340dce6a2d401d24fd37c6fe6c4f0ee0de8bfca2252d27"
+checksum = "c3083ce4b914124575708913bca19bfe887522d6e2e6d0952943f5eac4a74010"
 dependencies = [
  "futures-core",
  "futures-sink",
@@ -516,9 +516,9 @@ dependencies = [
 
 [[package]]
 name = "futures-core"
-version = "0.3.18"
+version = "0.3.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "629316e42fe7c2a0b9a65b47d159ceaa5453ab14e8f0a3c5eedbb8cd55b4a445"
+checksum = "0c09fd04b7e4073ac7156a9539b57a484a8ea920f79c7c675d05d289ab6110d3"
 
 [[package]]
 name = "futures-executor"
@@ -550,9 +550,9 @@ dependencies = [
 
 [[package]]
 name = "futures-sink"
-version = "0.3.18"
+version = "0.3.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "996c6442437b62d21a32cd9906f9c41e7dc1e19a9579843fad948696769305af"
+checksum = "21163e139fa306126e6eedaf49ecdb4588f939600f0b1e770f4205ee4b7fa868"
 
 [[package]]
 name = "futures-task"