Update composer requirement to PHP 7.4

[slashequip]

Description of the change

This MR bumps up the minimum required version of PHP, although no code changes are introduced this just ensures that the package is aligned with PHP's current minimum support offering.

As no code changes are introduced any consumers who want to use the latest version can use composer's --ignore-platform-reqs flag. Although I think this no-breaking-changes approach serves as a nice warning that latest and greatest code might be used in the future.

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

Fix #28

Checklists

Development

• [x] Lint rules pass locally
• [x] The code changed/added as part of this pull request has been covered with tests
• [x] All tests related to the changed code pass in development

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] "Ready for review" label attached to the PR and reviewers mentioned in a comment
• [ ] Changes have been reviewed by at least one other engineer
• [ ] Issue from task tracker has a link to this pull request

[tk26]

thanks for adding this @slashequip 🥇