Bump ServeHappy PHP constants to 7.3+

[SergeyBiryukov]

Background: #5257, #5279.

The SUPPORTED_PHP value in servehappy-config.php​ is currently at 7.3.

The SECURE_PHP and ACCEPTABLE_PHP values are at 7.2, last changed in [9959] / #5257.

Since security support for PHP 7.2 and active support for PHP 7.3 ​ended last December, these values should be updated. Some context:

• SUPPORTED_PHP is the lowest branch of PHP which is actively supported.
• SECURE_PHP is the lowest branch of PHP which is receiving security updates.
• ACCEPTABLE_PHP is the lowest branch of PHP which is still considered "acceptable" in WordPress. Sites with a version lower than this will see the ServeHappy dashboard widget urging them to update.

Based on the above, it seems like:

• SUPPORTED_PHP should be updated to 7.4.
• SECURE_PHP should be updated to 7.3.
• ACCEPTABLE_PHP should also be updated to 7.3, since we want users to have a secure version.

We can also discuss whether we want to bump the RECOMMENDED_PHP version to 8.0, however, given the large number of breaking changes there and unknown state of plugin compatibility, it might be a bit premature at this time.

[jrf]

I strongly support bumping the SUPPORTED_PHP, SECURE_PHP and ACCEPTABLE_PHP versions.

I also agree with @SergeyBiryukov that it is way too early to bump the RECOMMENDED_PHP version to 8.0. That should stay at 7.4 for the foreseeable future or at least until all ​PHP 8.0 related tickets have seen action or are solved, including ticket ​https://core.trac.wordpress.org/ticket/51553

[SergeyBiryukov]

In 10960:

API: ServeHappy: Bump the minimum acceptable PHP version to 7.3.

Sites with a version lower than this will see the ServeHappy dashboard widget urging them to update.

Additionally:

• Bump the lowest branch of PHP which is actively supported to 7.4.
• Bump the lowest branch of PHP which is receiving security updates to 7.3.

See ​https://www.php.net/supported-versions.php.

Follow-up to [9959].

Fixes #5727.