Fix static file caching is disabled in Airflow Webserver.

[VertexToEdge]

close #39344

Above Airflow 2.3.0, webserver's static file caching is disabled because of major updated Flask's breaking change. In Flask 2.0.x, default value of SEND_FILE_MAX_AGE_DEFAULT is changed from 12 hours to None on that version.
SEND_FILE_MAX_AGE_DEFAULT affects Expires Field in HTTP response header.
if it is None, Expires Field in HTTP response header is removed.

this pr add SEND_FILE_MAX_AGE_DEFAULT config for Flask explicitly in /airflow/www/app.py.
It enables static file caching in Airflow webserver.

I checked Airflow configuration references and searched SEND_FILE_MAX_AGE_DEFAULT in Airflow codebase, but there's no way to enable static file caching for airflow webserver's Flask.

Airflow 2.9.0: As-Is

Airflow 2.9.0: To-Be (Caching Worked)

Local test in /tests/www

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[boring-cyborg]

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[bbovenzi]

Nice catch! We could probably cache it for a few weeks. Assets will only change between airflow version updates.

[jscheffl]

Really cool catch. I can immediately see it locally and the UI feels immediately faster with this PR! THANKS!

(I think we could debate about the time, 12h sounds reasonable for me, still it will allow also proxy chains to cache files (if not HTTPS) - if somebody wants to have this nicer, still could be configurable. But 12h is a big improvement anyway!

[boring-cyborg]

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