Raise the minimum required version of MySQL to 5.5.5

[desrosj]

The minimum version of MySQL required to run WordPress has sat at 5.0 since 2011 (see [17523]).

MySQL 5.0 was initially made generally available in 2005 and reached its end of life (EOL) in January of 2012, making it unsupported and insecure for over 12 years. This is extremely old and this requirement is being raised.

Currently, only 0.4% of all WordPress sites are running either version 5.0 or 5.1 of MySQL.

Usage for MySQL 5.5 sits at just under 15%, making it the logical landing point. Of all sites running 5.5.x, 100% of them are running 5.5.5 or higher. utf8mb4 support was introduced in 5.5.3, and InnoDB was made the default storage engine over MyISAM in 5.5.5, making it worthwhile to use the 5.5.5 point release instead of any 5.5.x version (utf8mb4 is the preferred default character set).

Raises the minimum version of MySQL required to run WordPress to 5.5.5.

Trac ticket: core.trac.wordpress.org/ticket/60036

[desrosj]

In 57173:

Database: Raise the minimum required version of MySQL.

This raises the minimum version of MySQL required to run WordPress from 5.0 to 5.5.5.

MySQL 5.0 and 5.1 have long been unsupported and both reached end of life over 10 years ago. Combined usage for both versions sits at 0.4% of all WordPress sites.

Because 5.5 sits at just under 15% usage, 5.5 cannot be trimmed off at this time.

Of all sites running 5.5.x, 85% are running 5.5.5, and 100% are running 5.5.5 or higher. This makes it the logical landing spot.

Props johnbillion, sergeybiryukov, jorbin.
Fixes #60036.

Merged in https://core.trac.wordpress.org/changeset/57173