build(deps): update mockall requirement from 0.11.4 to 0.12.1 (#89)

Updates the requirements on [mockall](https://github.com/asomers/mockall) to permit the latest version.
- [Changelog](https://github.com/asomers/mockall/blob/master/CHANGELOG.md)
- [Commits](asomers/mockall@v0.11.4...v0.12.1)

---
updated-dependencies:
- dependency-name: mockall
  dependency-type: direct:production
...

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

flipt-engine/Cargo.toml

@@ -22,7 +22,7 @@ openssl = { version = "0.10", features = ["vendored"] }
 cbindgen = "0.26.0"
 
 [dev-dependencies]
-mockall = "0.11.4"
+mockall = "0.12.1"
 
 [lib]
 name = "fliptengine"