[BUGFIX] Extension Manager does not use calcAge

The extension manager does his own magic with calculating the age
of the last TER update. This can be done automatically with
BackendUtility::calcAge.

Resolves: #46962
Relates: #46524
Releases: 6.1, 6.0
Change-Id: I1917da7fa97e8692ee5bac799377e2a62ad14864
Reviewed-on: https://review.typo3.org/19634
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn