Use migration name or group name as page title

This is failing because of PHP 7.4.
Time to ditch this php version?

My latest PR still has some problems.
The group argument in some cases is just a string, so the title callback expecting an object as parameter won't work.
We should do different title callbacks for different routes.

Also, we get distinct items in the breadcrumb for:
- /admin/structure/migrate/manage/GROUP
- /admin/structure/migrate/manage/GROUP/migrations

We should put a different title into the breadcrumb than the page title.

There should be a distinct route setting for "breadcrumb title callback" or perhaps "navigation title callback".

We can set the title directly in a controller like:

return [
  '#title' => $migration->label(),
  'content' => $build,
];

But this does not work that easily for entity form pages where we don't own the controller.