Prepare 7.22.0

[wing328]

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.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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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

Prepare the 7.22.0 snapshot by bumping project versions, refreshing docs, and regenerating samples. Master now targets 7.22.0-SNAPSHOT and docs point users to 7.21.0 as the latest stable.

• Refactors
  • Set all module/plugin versions to 7.22.0-SNAPSHOT (pom.xml, Gradle, Mill) and regenerated samples with the new generator version.
  • Updated README to show 7.22.0 on master and adjusted CLI download/install commands to 7.21.0.
  • Updated release script to stop updating modules/openapi-generator-mill-plugin/pom.xml.

^{Written for commit 67a5a8f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3000 files

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed.