composer create-project should require PHP 7.3

Problem/Motivation

It is possible to run "composer create-project" to download a site using e.g PHP 7.2 but then only have it fail later on during the install when the codebase realizes that APIs from PHP 7.3 are not available.

Steps to reproduce

Run "composer create-project" with PHP 7.2.

Proposed resolution

Update the root composer.json file to list PHP 7.3 as a requirement.

Remaining tasks

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet

TBD