composer.json - Bump PHP minimum to 7.4

[totten]

Follow-up to #7 -- we don't need support 7.2 or 7.3 because:

• Current-stable CiviCRM requires 7.4+ (which is minimum for any distmaker-based deployments)
• The target-audience that sees this package is mostly composer-based -- and composer is clever enough to give them prior revisions.

cc @demeritcowboy

[totten]

Actually, I'm gonna go ahead with a quick merge. (There's a chance of provoking merge-conflict with #6; but #6 needs to be rebased anyway. Better to let one rebase do the job rather than leave the MC-risk open.)