Improve rendering of "System environment check"

The title should be wrapped in a <strong>-Tag

Change-Id: Idf76d37edbfaa437d1940d1e0db476fa41415bca
Resolves: #46546
Releases: 6.1
Reviewed-on: https://review.typo3.org/19175
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer