[CLEANUP] Use correct type hint for messages in install tool updates

The data type of customMessage has been changed to string a while ago
but the method signatures still have a mixed-type.
Normalize this over all updates to make things consistent again.

Resolves: #79739
Releases: master
Change-Id: I840cc6d217dafb1a83f0af5b7a978fdc60318d5d
Reviewed-on: https://review.typo3.org/51624


Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>