Remove the core key and update the core_version_requirement' key in the .info.yml file

Since the core key in .info.yml isn't anymore supported starting from Drupal 8.7.7 and semantic versions are supported only after Drupal 8.8.3, the .info.yml file needs to be changed as follows:

• core must be removed
• core_version_requirement must be changed to core_version_requirement: ^8.8 || ^9