Update webserver secret note in Helm chart NOTES.txt and prod guide

[amordoch]

---

Updated the Helm chart release notes to suggest setting a static API secret key instead of a webserver secret key for Airflow 3.0+. Correspondingly, updated the referenced production guide section to apply to both Airflow 3 and Airflow <3. If the note is only valid for Airflow <3 then I'm happy to change to that logic instead.

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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[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 (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
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[potiuk]

Thanks!

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.