Bump serde_json from 1.0.64 to 1.0.68 (#9783)

Bumps [serde_json](https://github.com/serde-rs/json) from 1.0.64 to 1.0.68.
- [Release notes](https://github.com/serde-rs/json/releases)
- [Commits](https://github.com/serde-rs/json/compare/v1.0.64...v1.0.68)

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

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

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

substrate/test-utils/client/Cargo.toml

@@ -16,7 +16,7 @@ codec = { package = "parity-scale-codec", version = "2.0.0" }
 futures = "0.3.16"
 hex = "0.4"
 serde = "1.0.126"
-serde_json = "1.0.55"
+serde_json = "1.0.68"
 sc-client-api = { version = "4.0.0-dev", path = "../../client/api" }
 sc-client-db = { version = "0.10.0-dev", features = [
     "test-helpers",