Use Display instead of ToString in Rust generators

[dani-garcia]

With the latest 1.78 release of Rust, Clippy has introduced a lint warning against the direct implementation of ToString, recommending instead to implement Display: https://rust-lang.github.io/rust-clippy/master/index.html#/to_string_trait_impl

This PR changes the uses of impl ToString in the Rust generator template for impl Display. It also updates the Rust samples. Note that because Display has a blanket implementation of ToString, the resulting enums will still implement ToString, so this doesn't break anything.

Tagging openapi-generator Rust committee, like the checklist mentions: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

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 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
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  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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[wing328]

thanks for the PR.

if no one else has further feedback on this change, I'll merge it over the weekend.

have a nice weekend.