Clarify recommendation on page_cache module

[stevector]

In our internal Slack we had a discussion of whether Drupal 9/10 sites need or should use page_cache module. I suspect it is made unnecessary by our CDN and that leaving it enabled causes more confusion than good.

• Pantheon Advanced Page Cache doesn't depend on page_cache: https://github.com/pantheon-systems/pantheon_advanced_page_cache/blob/2.x/pantheon_advanced_page_cache.info.yml
• Core's system module controls the setting of max-age as config
• FinishResponseSubscriber seems to control the application of that setting to the response object https://git.drupalcode.org/project/drupal/-/blob/11.x/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php#L253

[stevector]

Let's confirm and then open a PR.