Add PHP version support policy

[localheinz]

As the discussion comes up again and again (see #543 and #733), I propose that we discuss and adopt a PHP version support policy based on the release schedule of PHP versions.

For example, we could agree that we

• add support for a newly released PHP version within days of the initial release of a PHP version for actively supported branches of fakerphp/faker
• drop support for a PHP version that has reached its end of life one year after its end of life for actively supported branches of fakerphp/faker

Also see https://www.php.net/supported-versions.php.

[stale[bot]]

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[supgeek-rod]

Maybe we can maintain multiple versions at the same time, each version corresponds to a different php version.