Status page should link to D8/9 information for public files

Problem/Motivation

On /admin/reports/status page, if the "PUBLIC FILES DIRECTORY" settings are not proper it shows a message

Not fully protected
See https://www.drupal.org/SA-CORE-2013-003 → for information about the recommended .htaccess file which should be added to the directory to help protect against arbitrary code execution.

But when you go to the link https://www.drupal.org/SA-CORE-2013-003 → it feels like the whole page is targeted at Drupal 6 and 7. Either the documentation on the page needs to change or we should link it to relevant page.

Steps to reproduce

Set the private file path to say /tmp
$settings['file_private_path'] = '/tmp';
Navigate to /admin/reports/status

Proposed resolution

Use \Drupal\Component\FileSecurity\FileSecurity::htaccessLines() to show the user the recommend content.
Before

After

Remaining tasks

Review patch
Commit

User interface changes

/admin/reports/status has more clear message for private directory setup.

API changes

NA

Data model changes

NA

Release notes snippet

NA