[Super Cache] Show a warning when setting GC time over 24h

WordPress nonces last 24 hours. If a user has their Garbage Collection (or Preload frequency) set to a value over 24 hours, there is a chance that users will be served a stale nonce (e.g.: in a comment form), causing errors.

We should display a warning next to the setting if users enter a value over 24h to alert them to this possible issue.

Start by locating the Super Cache settings for Garbage Collection and Preload frequency, then inspect how those values are rendered and validated in the WordPress admin. Add a warning when either value exceeds 24 hours, and verify that the warning appears only for values above that threshold.

Written by the indexing model from the issue text.