chore(deps): update mockall requirement (#365)

Updates the requirements on [mockall](https://github.com/asomers/mockall) to permit the latest version.

Updates `mockall` to 0.15.0
- [Changelog](https://github.com/asomers/mockall/blob/master/CHANGELOG.md)
- [Commits](https://github.com/asomers/mockall/compare/v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: mockall
  dependency-version: 0.15.0
  dependency-type: direct:production
  dependency-group: library-deps
...

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library/Cargo.toml

@@ -73,7 +73,7 @@ oslog = "0.2.0"
 simple_logger = "5.0.0"
 
 [dev-dependencies]
-mockall = "0.14.0"
+mockall = "0.15.0"
 mockito = "1.2.0"
 mock_instant = "0.6.0"
 # Gives #[serial] attribute for locking all of our shorebird_init