Increase tested PHP versions

[BrianHenryIE]

Currently the test matrix is ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4'] with code coverage created on 7.4.

The plan is to support PHP 7.4 as long as WordPress does.

But I should add 8.5 to the test matrix. I wonder if setting config.platform.php to the matrix version, beyond today's composer.json, in require-dev, having "php": "^7.4|^8.0", would make any difference.

I don't think that is incorrect, but I am wondering would there be any unexpected side-effects if it was set strictly. Maybe some ancient package would be too old somehow. I don't know off-hand any PHP 5 (or lower) syntax that may have been deprecated in 7 and actually removed in 8.

Unrelated: I was considering including this library that hasn't been updated in 14 years skyzyx/phpt – it is so old it doesn't have an autoload key in its composer.json.

• https://www.phpinternalsbook.com/tests/phpt_file_structure.html
• https://php.github.io/php-src/miscellaneous/writing-tests.html#phpt-structure-details