Bump prost from 0.7.0 to 0.8.0 (#9560)

Bumps [prost](https://github.com/tokio-rs/prost) from 0.7.0 to 0.8.0.
- [Release notes](https://github.com/tokio-rs/prost/releases)
- [Commits](https://github.com/tokio-rs/prost/compare/v0.7.0...v0.8.0)

---
updated-dependencies:
- dependency-name: prost
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

substrate/Cargo.lock

@@ -7336,7 +7336,7 @@ dependencies = [
  "libp2p",
  "log 0.4.14",
  "parity-scale-codec",
- "prost 0.7.0",
+ "prost 0.8.0",
  "prost-build 0.7.0",
  "quickcheck",
  "rand 0.7.3",
@@ -8001,7 +8001,7 @@ dependencies = [
  "parity-scale-codec",
  "parking_lot 0.11.1",
  "pin-project 1.0.5",
- "prost 0.7.0",
+ "prost 0.8.0",
  "prost-build 0.7.0",
  "quickcheck",
  "rand 0.7.3",