[BUGFIX] Add version check to /install/index.php

This patch adds a missing PHP version check to the /install/index.php file which
exists only for the redirect to the install tool. Because of this missing version check
an installation without PHP 7.0 will fatal with an parsing error.

Resolves: #80496
Releases: master
Change-Id: Iec97021fb18ac7a4cfa0dcf1961675c3d58305ee
Reviewed-on: https://review.typo3.org/52193


Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>