Move migration code into a dedicated class

Problem/Motivation

Right now, all of the Layout Builder migration code is contained in a procedural function, _panelizer_migrate_to_layout_builder(). The business logic should be moved into a dedicated class which uses ContainerInjectionInterface for proper dependency injection.

Proposed resolution

Do that. No tests needed, since existing ones should not break and no actual business logic is being changed.

Remaining tasks

Patch and commit it once it passes tests.

User interface changes

None.

API changes

None. The migration class is not an API.

Data model changes

None.

Release notes snippet

None.