Use DatabasePlatform instead of Driver to check for MySQL/MariaDB

[andysh-uk]

Replace getDriver()->getName() call with getDatabasePlatform()->getName() to avoid a deprecation in Doctrine.

This returns mysql instead of pdo_mysql on MariaDB, at least, so the mb_strpos check should work just fine as-is.

[bobdenotter]

Thank you, @andysh-uk for this fix! Also thanks to you @Wieter, for helping to get to the root of the problem!

I love fixes like this! It seems like such an insignificant change, since it literally only changes one method call. The previous fix was a patch, fixing the symptoms. This little one-word change took several hours of work from multiple people, however it is a proper fix for an actual bug, and reduces workarounds further down the line! 🤗

[Wieter]

@bobdenotter you're welcome :)

This fix prevents future breakage due to depreciation, however it does not resolve issues #1971 and #1985 (outdated config file as introduced by composer install).

[andysh-uk]

I love your sentiments @bobdenotter (thank you) however the credit goes to @Wieter for finding the actual issue (with doctrine.yaml) - as you said, my previous fix was incorrect as it only worked around a problem, not resolved the root of the problem.

This PR is just something I noticed whilst working on this issue, that the Doctrine project have deprecated this method, believing it only to be used in their tests.