[TASK] Clean up workspaces-related code

This change most notably modifies EXT:workspaces
where various places do not throw a custom exception
anymore due to strict typing in PHP.

In addition, the language file "locallang_mod_user_ws.xlf"
has been removed, as it did not contain any useful labels
anymore, and were part of the "old" (pre-ExtJS) module.

In order to improve code readability, some methods have
been changed or renamed, or legacy comments have been
removed. Constants for internal stages are now used
everywhere.

Resolves: #102568
Releases: main
Change-Id: I30c78c4310c7dfb47f72eb212ef006f2ef937507
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82049


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Oliver Bartsch <bo@cedev.de>