Add Drupal 9 into the require section of composer.json

Problem/Motivation

Based on this other issue, it looks like Bootstrap Library is almost ready for Drupal 9!

I believe that if you add drupal 9 as an option to the composer.json file in this project you can get a nice little Compatible with Drupal 9 badge in the Project information section on the module page.

Below is a code sample from a module that has the badge.

"require": {
         "php": "^7.1",
        "drupal/core": "^8.7 || ^9"
   },

It also looks like it may possible to get the badge by adding the 'core_version_requirement' key in the modules info.yml file, which, in turn, will add the version to the require section ofcomposer.json. Example follows.

core_version_requirement: ^8 || ^9

Remaining tasks

* Update composer.json, or
* Update the info.yml file