chore(accordion): use rgb slash syntax instead of rgba

Use updated CSS syntax called out by the linter.

Author: jawinn

components/accordion/index.css

@@ -46,10 +46,10 @@
 	--spectrum-accordion-item-content-line-height: var(--spectrum-line-height-100);
 
 	/* Colors */
-	--spectrum-accordion-background-color-default: rgba(var(--spectrum-gray-900-rgb), var(--spectrum-background-opacity-default));
-	--spectrum-accordion-background-color-hover: rgba(var(--spectrum-gray-900-rgb), var(--spectrum-background-opacity-hover));
-	--spectrum-accordion-background-color-down: rgba(var(--spectrum-gray-900-rgb), var(--spectrum-background-opacity-down));
-	--spectrum-accordion-background-color-key-focus: rgba(var(--spectrum-gray-900-rgb), var(--spectrum-background-opacity-key-focus));
+	--spectrum-accordion-background-color-default: rgb(var(--spectrum-gray-900-rgb) / var(--spectrum-background-opacity-default));
+	--spectrum-accordion-background-color-hover: rgb(var(--spectrum-gray-900-rgb) / var(--spectrum-background-opacity-hover));
+	--spectrum-accordion-background-color-down: rgb(var(--spectrum-gray-900-rgb) / var(--spectrum-background-opacity-down));
+	--spectrum-accordion-background-color-key-focus: rgb(var(--spectrum-gray-900-rgb) / var(--spectrum-background-opacity-key-focus));
 
 	--spectrum-accordion-item-content-color: var(--mod-accordion-item-content-color, var(--spectrum-body-color));
 	--spectrum-accordion-divider-color: var(--mod-accordion-divider-color, var(--spectrum-gray-200));