Add PHP version constraint in composer.json "require" section

[Potherca]

Currently this package does not state which PHP versions it supports.

Based on CI/CD runs, the following seems to be rue:

@poef Which versions should remain supported?

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Actions

• Define which versions should be supported
• Add "PHP" entry to "require" section of composer.json
• Update PHP versions in .github/workflows/php-compat.yml Remove support for PHP < 7.3 from php-compat.yml. #16

[poef]

Since PHP 7.2 (and 7.3 for that matter) are no longer in the supported versions of PHP list, I think we're good here. Anything before 7.3 doesn't need to be supported.

[Potherca]

FYI: I came across the lack of php in the "require" section using this totally userfriendly and intuitive tool: https://blog.pother.ca/php-8-yet/