[BUGFIX] Make sure EXT:seo can be used without EXT:dashboard installed

Move widget registration to Services.php to allow checking for the
existence of WidgetRegistry to only add the widget if EXT:dashboard is
available.

Resolves: #99813
Releases: main
Change-Id: I3157784944cdf16ae091a8ad90da59894c94104b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77686


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>