Make Helm chart version optional in bug reports

[IcebreakerSA]

Bug reports unrelated to the Helm chart currently require reporters to select a chart version, usually Not Applicable. Making the field optional removes an unnecessary step while preserving the issue category, Airflow version, reproduction details, and Code of Conduct as required fields.

The existing Not Applicable default remains available for reporters using the Helm chart.

closes: #55840

Testing:

• prek run yamllint --files .github/ISSUE_TEMPLATE/1-airflow_bug_report.yml
• prek run check-airflow-bug-report-template --files .github/ISSUE_TEMPLATE/1-airflow_bug_report.yml
• codespell .github/ISSUE_TEMPLATE/1-airflow_bug_report.yml
• Manual-stage static checks

No runtime tests are needed because this only changes GitHub Issue Form validation.

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Was generative AI tooling used to co-author this PR?

• Yes — OpenAI Codex (GPT-5)

Generated-by: OpenAI Codex (GPT-5) following the guidelines

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Drafted-by: OpenAI Codex (GPT-5) (no human review before posting)

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
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