[BUGFIX] Use config.extTarget for menu links to external URLs

Resolves: #78151
Releases: master
Change-Id: I65356a01f76d1c693a45e4ba84884f6ad01cdf65
Reviewed-on: https://review.typo3.org/54754


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Kummer <typo3@enobe.de>
Tested-by: Joerg Kummer <typo3@enobe.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Henning Liebe <h.liebe@neusta.de>
Tested-by: Henning Liebe <h.liebe@neusta.de>
Reviewed-by: Riccardo De Contardi <erredeco@gmail.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>