[Maintenance]: Update minimum PHP version to PHP7.4

[ssddanbrown]

Describe the feature you'd like

To align with the PHP release/support cycle, and prepare for an eventual upgrade to Laravel 9, we should drop PHP7.3 support and make PHP7.4 the new supported minimum.

PHP7.3 wasn't really part of many "LTS" operating system packages, and official PHP security support has now dropped. In addition, many third party packages have already dropped PHP7.3.

Doing this upgrade now assures a steadier change in requirements rather than needing to drop two versions at once in the future.
Our last change to the PHP requirement was in v21.04. We should make this change in the next feature release or that thereafter, Especially while I've got the extra time to support where needed.

[ssddanbrown]

• Changes ready in Updated minimum php version from 7.3 to 7.4 #3245.
• Have checked devops/website repos for script usages, cannot see any scripts we need to update or upgrade paths we need to support.
• Just needs requirements and update notes additions/changes.