Support PHP 7.4 \(bunq/sdk_php#211\)

[HVSoftware]

This MR will update the composer.json to support PHP 7.4.

This PR closes/fixes the following issues:

• Upgrade to PHP 7.4#
  • [ ] Tested
• Update phpro/grumphp package#
  • [ ] Tested
• Update phpstan/phpstan package#
  • [ ] Tested

[HVSoftware]

This MR will be closed in favor of https://github.com/bunq/sdk_php/pull/212. This MR has also support of PHP 8.0.