[TASK] Move update functionality to BackendUserSessionRepository

As annotated in SwitchBackUserHook.php, the update method is moved
into the BackendUserSessionRepository file.

Resolves: #57012
Releases: 6.2
Change-Id: I5e127b8e71585e64bd99ae3175655b058162da83
Reviewed-on: https://review.typo3.org/28487
Reviewed-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Pascal Dürsteler
Tested-by: Pascal Dürsteler
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind