Add the 'core_version_requirement' key in info.yml and composer.json files

Problem/Motivation

Add core_version_requirement: ^8 || ^9 to admin_toolbar.info.yml to designate that the module is compatible with Drupal 9. See https://www.drupal.org/node/3070687.

Proposed resolution

Add the 'core_version_requirement' key in info.yml and composer.json files.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.