Bump default PHP version to 7.4

[ampersarnie]

Description

As PHP 7.3 reached End of Life 10 days ago (6th Dec 2021) we should ensure that we are at least using the minimum supported version as the install default.

https://www.php.net/supported-versions.php

Change Log

• Changed default PHP version to target 7.4.

Types of changes (if applicable):

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

Checklist (if applicable):

• [x] Meets provided linting standards.