[BUGFIX] Taskcenter: Removed call to deprecated method render()

Calls of FlashMessage->render() are replaced with appropriate code.

Resolves: #72447
Releases: master, 7.6
Change-Id: I2163fe8fea9b654034fc5a45d05a21ae8c99472c
Reviewed-on: https://review.typo3.org/45449


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>