Enhancement: Add support for PHP 8.0 and updated minimal php version to 7.3

[EJTJ3]

Due to the deprecated composer v1, and supported php versions we need to upgrade some of the dependencies for this bundle:

https://blog.packagist.com/deprecating-composer-1-support/

What should happen:

1. Upgrade minimal PHP version to 7.3 | 8.0 (https://www.php.net/supported-versions.php)
2. Upgrade phpro/grumphp to version 1.*
3. Upgrade phpunit/phpunit to version 9
4. Upgrade phpstan/phpstan to version 0.12
5. Upgrade guzzlehttp/guzzle to version 7 (Upgrade to Laravel 8 not possible due to "old" guzzlehttp/guzzle #204)
6. Replace jakub-onderka/php-parallel-lint with php-parallel-lint/php-parallel-lint
7. Remove sebastian/phpcpd (deprecated)
8. Remove sensiolabs/security-checker (deprecated)

What happens:

1. Losing support for php7.1 & php7.2
2. Adding support for php8.0
3. Adding support for composer v2

SDK version and environment

• Tested on 1.14.1
• Sandbox
• Production