fix: make Playback Rate control work better with screen readers (vide…

…ojs#7193)

Add an aria-describedby to the playback rate button.

Improves behavior of videojs#7121.

src/js/control-bar/playback-rate-menu/playback-rate-menu-button.js

@@ -26,6 +26,8 @@ class PlaybackRateMenuButton extends MenuButton {
   constructor(player, options) {
     super(player, options);
 
+    this.menuButton_.el_.setAttribute('aria-describedby', this.labelElId_);
+
     this.updateVisibility();
     this.updateLabel();
 
@@ -42,8 +44,11 @@ class PlaybackRateMenuButton extends MenuButton {
   createEl() {
     const el = super.createEl();
 
+    this.labelElId_ = 'vjs-playback-rate-value-label-' + this.id_;
+
     this.labelEl_ = Dom.createEl('div', {
       className: 'vjs-playback-rate-value',
+      id: this.labelElId_,
       innerHTML: '1x'
     });