Bug 37992: Make memberresultst thead sticky

1. Run empty patron search
2. Assuming there are enough results, when scrolling down the #searchheader with "select all" button would get hidden. But no, it's sticky so it follows you and stays visible. This is expected.
3. The table header (thead) with filters for each column is not sticky. This is a regression.
(It was sticky in 23.11, but not anymore in 24.05 and main)
4. APPLY PATCH and regenerate CSS.
5. The thead should again be sticky.
6. Test in at least Chrome and Firefox.

Notes:
-Setting top to 40px, which covers the height of the #searchheader
-To avoid border styles not working with the sticky element I am unsetting border-collapse.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Brendan Lawlor <blawlor@clamsnet.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 481f8ce3db)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>

koha-tmpl/intranet-tmpl/prog/css/src/staff-global.scss

@@ -1929,6 +1929,14 @@ li {
     padding: 20px;
 }
 
+#memberresultst {
+    border-collapse: unset;
+    thead {
+        position: sticky;
+        top: 40px;
+    }
+}
+
 #searchresults {
     background-color: #FFFFFF;
     margin-bottom: 1rem;