Drop `boot_files` from schema and data files

[SchoolGuy]

This request is related to cobbler/cobbler#3439.

During the investigation of the Item inheritance schema an important thing discovered:

• boot_files and template_files have the same functionality. As the second one is a superset of the first one, I would like to have the first one removed.

I have double checked double-checked this in the following places:

• Cobbler
• Koan
• Terraform Provider

As these are the only places where this could be used in the past, I wish to remove the in the title-mentioned attribute without replacement.

To enable people to work with the old schemas, the CLI should retain functionality for both old and new schemas. A migration of the v2 to v3 schema (because the change is breaking) is desired.

Edit: Remove second attribute as it is not present in the JSON schema.

[SchoolGuy]

The functionality of fetchable_files was introduced with cobbler/cobbler@253f7a9 but the same can be achieved via template_files once the code is fixed.