Do not allow core: 8.x in info.yml file in submodule

Problem/Motivation

Adding both core and core_version_requirement with anything other than core_version_requirement: ^8 || ^9 || ^10 will result in an exception. This is not valid because the core: key would include versions that do not recognize the core_version_requirement: key.

Found this core key in submodule.

Steps to reproduce

1. Install the module
2. Go to this xmlrpc_example submodule directory
3. Check .info file.

Proposed resolution

For example, a module that is compatible with Drupal 8 versions after Drupal 8.8.0 and also Drupal 9 will need an info.yml file like this:

name: My Module
type: module
core_version_requirement: ^8 || ^9 || ^10