[TASK] Unify dashboard widget registration

All widgets should be registered the same way to
increase consistency in the core file. The second
registration variant is documented in the corresponding
rst file anyway.

Resolves: #90502
Releases: master
Change-Id: I3e12890cecb130f4ea6815e01501691c5081735b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63387


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>