[BUGFIX] Add missing borders below tablesort th elements

While fixing an issue with sticky icons, the borders
below th elements with "position: relative" got
removed. This is fixed by adding the css property
"background-clip: padding-box" to them, as this
prevents the drawing of background beneath
borders.

Resolves: #95605
Related: #95455
Releases: master
Change-Id: Ic1773aae97f466a894fb80c7e3bff35233f76a5c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71596


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>

Build/Sources/Sass/libs/_tablesort.scss

@@ -2,6 +2,7 @@
 th[role=columnheader]:not([data-sort-method="none"]) {
     cursor: pointer;
     position: relative;
+    background-clip: padding-box;
 
     &:after {
         content: '';