fix(aria/accordion): rename value to panelId for trigger and panel

[andrewseguin]

// Before
<div ngAccordionGroup [(values)]="openedPanels">
  <h2>
    <button ngAccordionTrigger value="a11y">
      Title
    </button>
  </h2>
  <div ngAccordionPanel value="a11y">
    <ng-template ngAccordionContent>
      Content
    </ng-template>
  </div>

// After
<div ngAccordionGroup [(expandedPanels)]="openedPanels">
  <h2>
    <button ngAccordionTrigger panelId="a11y">
      Title
    </button>
  </h2>
  <div ngAccordionPanel panelId="a11y">
    <ng-template ngAccordionContent>
      Content
    </ng-template>
  </div>

[andrewseguin]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 221402f
• 21.0.x: ec6045b