[BUGFIX] Hardcoded "admin" user in Installer template

The hardcoded username "admin" is removed from the HTML template
since the username can now be freely chosen.

Resolves: #53905
Releases: 6.2
Change-Id: I1c2846e60ee57f5e220405a9217fc1023d4d8bee
Reviewed-on: https://review.typo3.org/25668
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind