Bump Rust edition to 2021 and a few crates

[maxnachlinger]

Bump Rust edition to 2021 and a few crates. Fixes: https://github.com/OpenAPITools/openapi-generator/issues/18609

PR checklist

• [x] Read the contribution guidelines.
• [x] 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.
• [x] 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. 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*. IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• [x] 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. can you please take a look at https://github.com/OpenAPITools/openapi-generator/actions/runs/9002489911/job/24730910208?pr=18608 when you've time?

[maxnachlinger]

@wing328 - I opened this PR too quickly, sorry for the churn. Fixing the build now :(

[wing328]

Please take our time. Appreciate the PR to bump the dependencies to newer versions.

If you need help, let me know. (ping me via Slack)

[maxnachlinger]

@wing328 - PR is ready for a review

[wing328]

lgtm. thanks for the PR.