[BUGFIX] Update wording in Core Upgrade dialog

This change updates the text in the Core Upgrade dialog and fixes a
typo as well as introduces some basic styles.

Resolves: #86274
Releases: master
Change-Id: Ie6433a9fa3a5f8b8d1c808955872dda91d10788a
Reviewed-on: https://review.typo3.org/58292


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>