Bump serde from 1.0.204 to 1.0.208

Bumps [serde](https://github.com/serde-rs/serde) from 1.0.204 to 1.0.208.
- [Release notes](https://github.com/serde-rs/serde/releases)
- [Commits](serde-rs/serde@v1.0.204...v1.0.208)

---
updated-dependencies:
- dependency-name: serde
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Cargo.toml

@@ -11,7 +11,7 @@ rust-version = "1.60.0"
 
 [dependencies]
 arrayvec = { version = "0.7", default-features = false }
-serde = { version = "1.0.204", default-features = false, optional = true }
+serde = { version = "1.0.208", default-features = false, optional = true }
 parity-scale-codec-derive = { path = "derive", version = ">= 3.6.8", default-features = false, optional = true }
 bitvec = { version = "1", default-features = false, features = [ "alloc" ], optional = true }
 bytes = { version = "1", default-features = false, optional = true }