Steps to reproduce the issue

The PlgSystemHttpHeaders callbacks (setHttpHeaders, applyHashesToCspRule) are not correctly implementing the SubscriberInterface callbacks interface.

This is not an error on PHP, but in the future, it will generate a fatal error.

Expected result

These are the fixed signatures:

• public function setHttpHeaders(Event $event): void
• public function applyHashesToCspRule(Event $event): void

Actual result

System information (as much as possible)

Additional comments