Moved "Overview" Local Task from Content Moderation module to System module

The "Overview" local task, previously defined in the content_moderation module, is now moved to the system module in core. This change centralizes the content administration interface, making it more consistent across different sites regardless of whether content_moderation is enabled.

Change Description:

The local task for the content overview page /admin/content is now provided directly by the system module. This ensures that the task is available even when the content_moderation module is not enabled, providing a more consistent user experience.

Impacts:

• Modules relying on system.admin_content as a parent route will continue to function without any changes.
• Sites not using content_moderation will now have the "Overview" local task available by default.
• Third-party modules that define their own "Overview" task will need to be updated to avoid duplication since this task is now provided by the system module.

Code Examples:

If your module defines an "Overview" task like this:

content_moderation.content:
  title: 'Overview'
  route_name: system.admin_content

You should remove or update this definition to avoid duplication. The task is now automatically provided by the system module, so it may no longer be necessary to define it manually in your module.

Examples:

Before: The "Overview" task was only available when content_moderation was enabled.

After: The "Overview" task is available by default, provided by the system module.