Test 6.0.38 backports from 6.1 branch

For all users still using PHP 7.3 – 7.4:

I back-ported all features not breaking the upgrade procedure from the 6.1 branch to 6.0.38.

I run a few tests locally with an old XAMPP supporting PHP 7.3, but it's almost certain I've missed something.

Please report back if something breaks.

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