[5.2][Events] Use event classes for System plugins

[Fedik]

Summary of Changes

Update system plugins to use SubscriberInterface and event classes.

Testing Instructions

Check following system plugins should work as before:

accessibility
actionlogs
fields
highlight
jooa11y
languagecode
log
privacyconsent
remember
skipto
stats

Actual result BEFORE applying this Pull Request

Works

Expected result AFTER applying this Pull Request

Works

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: New Event classes Manual#177
• No documentation changes for manual.joomla.org needed

[richard67]

@Fedik System tests are failing in Drone when trying to delete a user and waiting for the content "User deleted." appearing in the in the system-message-container div.