Bump backoff from 0.2.1 to 0.4.0

Bumps [backoff](https://github.com/ihrwein/backoff) from 0.2.1 to 0.4.0.
- [Release notes](https://github.com/ihrwein/backoff/releases)
- [Commits](https://github.com/ihrwein/backoff/compare/v0.2.1...v0.4.0)

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

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

bridges/relays/finality/Cargo.toml

@@ -9,7 +9,7 @@ description = "Finality proofs relay"
 [dependencies]
 async-std = "1.6.5"
 async-trait = "0.1"
-backoff = "0.2"
+backoff = "0.4"
 bp-header-chain = { path = "../../primitives/header-chain" }
 futures = "0.3.5"
 log = "0.4.17"

bridges/relays/utils/Cargo.toml

@@ -10,7 +10,7 @@ ansi_term = "0.12"
 anyhow = "1.0"
 async-std = "1.6.5"
 async-trait = "0.1"
-backoff = "0.2"
+backoff = "0.4"
 isahc = "1.2"
 env_logger = "0.8.2"
 futures = "0.3.5"