This PR introduces a basic AdminNotice model to store these notices. Admin notices are categorized by their source/type (currently only notices from problem check.) They also have a priority.
In #26122 we promoted all problem checks defined as class methods on AdminDashboardData to their own first-class ProblemCheck instances.
This PR continues that by promoting problem checks that are implemented as blocks as well. This includes updating a couple plugins that have problem checks.