[TASK][EM] More readable "time since last update"-strings

The extensionmanager now shows the time elapsed since the last update,
not the time of the last update. The time of the last update is still
shown in a tooltip. Some english localizations have changed.

Resolves: #46524
Related: #39921
Releases: 6.1
Change-Id: I2d5c6e8099e6973826debfcf2cbb597c35bb499c
Reviewed-on: https://review.typo3.org/19116
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn