* MySQL 5.6 has reached EOL (“End of Life”) in February 2021. The recommended minimum is bumped to 5.7 for now.
* MariaDB 10.2 has reached EOL in May 2022. The recommended minimum is bumped to 10.3 for now.
This commit brings the Site Health recommendations in line with `readme.html`.
Includes:
* Adding two unit tests to ensure the SQL server versions recommended by Site Health match `readme.html`.
* Consistently declaring the recommended and required versions as the `WP_Site_Health` class properties.
* Renaming some pre-existing private properties for clarity.
Follow-up to [44986], [52319], [52358], [52420], [52424], [53431], [53433], [53435], [meta11407], [meta11866].
See #55791, #meta5999, #meta6322.
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