Update the value for the core_version_requirement key and remove the core key

The social_pwa.info.yml file contains the following line.

core_version_requirement: ^8 || ^9

The activity_send_push.info.yml file contains the following lines.

core: 8.x
core_version_requirement: ^8 || ^9

Since this branch is using semantic version, it won't be compatible with any Drupal version earlier than drupal 8.8.3. The value for core_version_requirement needs to be updated to core_version_requirement: ^8.9 || ^9.
Changing the value for core_version_requirement also requires to remove the core key.

The same change isn't necessary for the 8.x-1.x branch, since at least a branch needs to be compatible with every Drupal 8 version (despite they aren't supported).