[TASK] HR - single horizontal rule (CSS)

Make HR a single line instead of two lines.

Change-Id: I433d025eb91afc2fd35e794089f330e77d42a966
Resolves: #55486
Releases: 6.2
Reviewed-on: https://review.typo3.org/27175
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters