update accordion item tabindex based on disabled state

Author: os-davidlourenco

core/src/components/accordion/accordion.tsx

@@ -533,6 +533,7 @@ export class Accordion implements ComponentInterface {
 
           'in-accordion-group-expand-inset': hostContext('.accordion-group-expand-inset', this.el),
         }}
+        tabindex={disabled ? -1 : undefined}
       >
         <div
           onClick={() => this.toggleExpanded()}