container: false on container_* attributes hides the field on the menu link form

Problem/motivation

The config form accepts container: false on an attribute with the container_ prefix, for example:

container_foo:
  container: false

ConfigForm::validateForm() always strips the prefix and validates foo. The menu link form only strips it for container attributes, so it validates container_foo, which fails because of the underscore, and silently skips the field. Editors never see it. Drupal core would drop container_foo on the <a> anyway.

Proposed resolution

Reject container: false on container_* names in ConfigForm::validateForm() with a message to rename the attribute (for example container_foo to foo). Add a functional test.