Add TYPO3 8 compatible FlashMessage Rendering in update script

[thomaszbz]

MetaSEO version: 3.0.0
TYPO3 version: 8

PR from @ipf

#501 fixes the issue, that executing the updater script in the TYPO3 8
backend results in a fatal error, because the FlashMessage handling has
been changed.

[SvenDeKa]

I patched this. I don't know when I'll find the time for contributing via github, so here is my patch for 8.7 in the meantime

class.ext_update.php.t3v8.txt
(sry, you cant attach *.patch-files here)

[typoworx-de]

Simply patch the Update-Script like this:

File: metaseo/class.ext_update.php

    protected function generateOutput()
    {
        $output = '';

        foreach ($this->messageList as $message) {
            $flashMessage = GeneralUtility::makeInstance(
                \TYPO3\CMS\Core\Messaging\FlashMessage::class,
                $message[2],
                $message[1],
                $message[0]
            );
            $output .= $flashMessage->__toString();
        }

        return $output;
    }

Change:
$output .= $flashMessage->render(); --> $output .= $flashMessage->__toString();