[Java] Apply Same License-Header in All Java Generators

[Chrimle]

Applies the same License-header to all generated Java classes, regardless of which Java generator is used. This is already the case for the still-maintained Java generators. For other generators, the resulting Java classes will have slightly different license headers. These are; apex and java-undertow-server. This should be considered "internal refactoring", and is only aimed at improving maintainability.

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

Unifies the license header across all Java generators by switching to a single shared template and removing generator-specific licenseInfo.mustache files. This standardizes generated headers and reduces maintenance; changes are limited to comment headers.

• Refactors
  • Removed per-generator licenseInfo.mustache files and routed all Java generators to the shared license header.
  • Matches the header already used by maintained Java generators.
  • No behavioral changes; only header comments in generated files may differ.

^{Written for commit 700a0d2. Summary will update on new commits.}

[cubic-dev-ai]

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