[FEATURE] Add log message for "Switch User action"

If an admin user switches to another be_user account via the "Switch User"
action in the "Backend users" module, the action is now logged to the
sys_log.

Resolves: #78432
Releases: master
Change-Id: I0a9cb35e2d8734eb86ff475afeeac821a2f2a78a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60824


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Guido Schmechel <guido.schmechel@brandung.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>