[rust-server] Update mockall to 0.14

[dsteeley]

Update a dependency through a non-semver compatible change. Mockall 0.13->0.14 in the rust-server generator.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
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• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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Summary by cubic

Updated the rust-server generator to use mockall 0.14 and refreshed the sample Cargo.toml files. This aligns generated servers with the latest mockall release.

^{Written for commit 11d9ac4. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 8 files