Move shared git push template to _common

[ChihweiLHBird]

Summary

Consolidate 38 byte-identical git_push.sh.mustache templates into the existing _common template directory.

Generators continue to resolve the script through the standard common-template fallback, while generator-specific variants remain unchanged.

PR checklist

• Read the contribution guidelines.
• 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.
• 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

Consolidated 38 duplicate git_push.sh.mustache templates into the shared _common/git_push.sh.mustache to reduce duplication and simplify maintenance. Generators continue to resolve the script via the common-template fallback; generator-specific overrides are unchanged.

^{Written for commit 8abfd56. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 38 files

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