Prepare v7.20.0 release

[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.
  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.
• 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)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Prepare the v7.20.0 release by bumping project versions to 7.20.0-SNAPSHOT, updating docs to point to 7.19.0 as the latest stable, and regenerating samples with the new generator version.

• Dependencies
  • Set parent and module POMs to 7.20.0-SNAPSHOT.
  • Updated Gradle plugin properties and sample projects to 7.20.0-SNAPSHOT.
  • Updated README and docs: release table marks 7.20.0 as upcoming and 7.19.0 as latest stable; CLI download URLs and version-manager/pip examples use 7.19.0.
  • Maven/Gradle plugin usage in docs set to 7.19.0; example plugin configs under modules reference 7.20.0-SNAPSHOT.
  • Regenerated sample clients to reflect generator version 7.20.0-SNAPSHOT.

^{Written for commit 369ffc2. Summary will update on new commits.}

[cubic-dev-ai]

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