Super Cache: fix how the plugin uses apache_request_headers

[donnchawp]

If a site has disabled the function apache_request_headers() then WPSC will not display the "Rejected User Agents" textarea on the settings page. It would also cause a fatal error because the code checked if the function existed, before creating a fallback of the same name.

Proposed changes:

• Create a function wpsc_apache_request_headers() that is a fallback for that function, and it checks if the original function exists and is callable before calling it. Otherwise, it extracts the headers.
• Remove the apache_request_headers() checks that were added 6 years ago to hide the Rejected User Agent settings.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• If you're using the Jetpack Docker container, login to it with jetpack docker sh
• Edit the php.ini of your server. If the above container, it'll be /etc/php/8.2/fpm/php.ini
• Look for "disable_functions =" and add apache_request_headers at the end.
• Restart PHP with service php8.2-fpm restart
• Go to the WPSC settings, and enable "304 Browser caching".
• Enable debugging in the plugin and watch the logs in a new browser tab.
• Open a new private window and load the front page of your site.
• You should see a log entry: "304 browser caching not possible as timestamps differ"
• Reload the page, and you may see this: "Send 304 Not Modified header."
• You could also error_log $remote_mod_time in wp-cache-phase2.php on line 289 and line 293 to make sure they match.
• Don't forget to reverse changes to php.ini and restart PHP.

[github-actions]

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[dilirity]

Really appreciate the thorough testing instructions!

Testing was successful and code looks good.