[FEATURE] Allow to provide name for new admin users in ext:install

The field `realName` has been added to the "Create Administrative User"
modal in ext:install, so it is possible to provide the name of a new
admin user.

The notice in the header of the model has been removed, since it is
superfluous now.

Resolves: #99584
Releases: main
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: I6732bfe4d664f200b2bcc3471c6f0ffd9859365b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77455


Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>