[BUGFIX] EM manual links open in the same window

If you open an extension in the get menu (show all versions), a link to
the manual is rendered.
It currently opens with target _top.

Change it to _blank to force opening the link in a new window.

Fixes: #46983
Releases: 6.0,6.1
Change-Id: Ie4f8bd09e94addc453b13a4fcc9745c39f14fe2c
Reviewed-on: https://review.typo3.org/19669
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn