Update mpdf dependency to version 8

Added support for PHP 7.3 and 7.4

This is a quick an dirty switch to the current MPDF devel version

This makes the plugin working on PHP 7.1 again, however I did not look
deeply in how much the API changed and just tried to quickly

This is a quick an dirty switch to the current MPDF devel version

This makes the plugin working on PHP 7.1 again, however I did not look
deeply in how much the API changed and just tried to quickly make the
whole thing work again.

I opted for a composer based install of the library to have the proper
dependencies installed. I also checked in the result to have the plugin
easily install via the extension manager.

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