Add --font-weight-bold and set previous bold to 601 (#24307) (#24331)

Backport #24307

Fix #24305

According to MDN, "bold" starts from 700, some fonts do not provide
"bolding" for weight 600

Manually backport, no CSS conflict.

web_src/css/shared/issuelist.css

@@ -37,7 +37,7 @@
   color: var(--color-text);
   font-size: 16px;
   min-width: 0;
-  font-weight: 600;
+  font-weight: var(--font-weight-bold);
 }
 
 .issue.list > .item .issue-item-top-row a.index {