[Extensions] Alert to notify an admin that modules need to be upgraded
At this page, there should be an alert to notify an admin that modules need to be upgraded (similar to how there are already alerts for "Security analyser warnings" and "Developer notices (development mode on)":
/index.php?module=administration&type=admin&func=adminpanel
Right now, the indication is buried at the Extensions module list page, under "Actions":
/index.php?module=extensions&type=admin&func=view&startnum=0
Postponing, as a clean solution should be done together with #909. Also performance aspects must be considered during implementation.
Closing since we are now jusing Composer and Flex like Symfony default. Hence, automatic upgrade notifications are possible by using Dependabot or similar services.