Bump the recommended PHP version to 7.4

[SergeyBiryukov]

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

It was bumped 18 months ago in [7990], following the 7.3 release on 6 Dec 2018. Per the comment, it's supposed to be "The latest branch of PHP which WordPress.org recommends".

Per ​discussion with @Clorith on Slack, given that ​PHP 7.3 support ends in 5 months, now seems like a good time to recommend 7.4 instead.

The previous approach was to follow the recommendation on the ​Download page, however it just displays the RECOMMENDED_PHP value now as of [8066]. Before that, it was bumped to 7.2 two years ago: [7366].

Some things to note:

• The recommended version is mentioned on ​Download and ​Requirements pages.
• Bumping RECOMMENDED_PHP to 7.4 also means that WP core would display a "Your site is running an older version of PHP" message for <= 7.3 in Site Health. However, this does not trigger the Dashboard nag, as that widget depends on the ACCEPTABLE_PHP value, which is currently set to 7.2 as of [9959].

[SergeyBiryukov]

In 9975:

API: ServeHappy: Bump the recommended PHP version to 7.4, the latest stable branch.

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

Follow-up to [9959].

Props Clorith.
Fixes #5279. See #5257.