Fix EZP-23403: Fix implementation of Persistence\Solr\Slot\MoveUserGroup slot

This PR resolves issue https://jira.ez.no/browse/EZP-23403

It turned out `MoveSubtree` slot also need to be fixed, as it was reindexing all Locations of the Content in the moved subtree, while only moved Locations needed reindexing.
`MoveUserGroup` slot is fixed by extending `MoveSubtree` slot and using its implementation.

Testing: manual testing

Followup: remove UserService::moveUserGroup() method: https://jira.ez.no/browse/EZP-23699
You could move the location index at the beggining, and exit if the content has already been processed:

```
            $locationSearchHandler->indexLocation(
                $locationHandler->load( $locationId )
            );

            if ( isset( $processedContentIdSet[$contentId] ) )
            {
                continue;
            }
            $this->persistenceHandler->locationSearchHandler()->indexLocation( $location );
            $contentSearchHandler->indexContent(
                $contentHandler->load(
                    $contentId,
                    $contentHandler->loadContentInfo( $contentId )->currentVersionNo
                )
            );

            // Content could be found in multiple Locations of the subtree,
            // but we need to (re)index it only once
            $processedContentIdSet[$contentId] = true;
```