bug(mat-stepper): a11y disabled state #23088

Reproduction

Steps to reproduce:

1. Turn on VoiceOver or any screenreader.
2. Go to a linear stepper element with more non-optional steps.
3. Hover the second step in the stepper.

Expected Behavior

The disabled state should be announced. aria-disabled could solve this.

Actual Behavior

There is no information on disabled state.

Environment

• Angular: 12
• CDK/Material: 12
• Browser(s): Any
• Operating System (e.g. Windows, macOS, Ubuntu): Any