fix(compat): add element selector for cdk-accordion (#5954)

Author: andrewseguin

src/lib/expansion/accordion.ts

@@ -19,7 +19,7 @@ let nextId = 0;
  * Directive whose purpose is to manage the expanded state of CdkAccordionItem children.
  */
 @Directive({
-  selector: '[cdk-accordion]',
+  selector: 'cdk-accordion, [cdk-accordion]',
 })
 export class CdkAccordion {
   /** A readonly id value to use for unique selection coordination. */