[BUGFIX] Disabled backend users shown for workspace stage change

On changing the stage of a workspace version, an accordant
dialog is shown with recipients to be informed about this
action. However, disabled backend users are shown in this
dialog as well.

Resolves: #54925
Releases: 6.2
Change-Id: Iefae9e35e9c680783769260e8c3e38180bde5703
Reviewed-on: https://review.typo3.org/26773
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters