Fix DBTabItem selected state and aria-selected attribute

Now also sets aria-selected=true|false correctly which improves screen reader behaviour.

Author: milan-w

.changeset/sharp-pumas-obey.md

@@ -0,0 +1,6 @@
+---
+"@db-ux/core-components": patch
+---
+
+fix: set DBTabItem internal state `_selected` correctly
+- now also sets aria-selected=true|false correctly which improves screen reader behaviour