bug(stepper): aria-disabled not properly updated for step header buttons in linear steppers

#### Reproduction

Example of a linear stepper with this issue: https://stackblitz.com/angular/vbkvxdpeoml?file=src%2Fapp%2Fstepper-overview-example.ts

Steps to reproduce:
1. Enable linear mode
2. Fill out first step
3. Go back to step header using keyboard navigation
4. Move focus to next step

#### Expected Behavior

The screen reader should read aloud the content of the button but not mention anything about being disabled since clicking the button will work.

#### Actual Behavior

The screen reader says the button is disabled.

This is because aria-disabled attribute is still set on the button, but shouldn't be anymore since the step is now complete.

#### Environment

- Angular: 12.2.4
- CDK/Material: 12.2.4
- Tested on Chrome with NVDA, ChromeVox, and VoiceOver.

Inizia con l’esempio StackBlitz collegato in src/app/stepper-overview-example.ts e riproduci la sequenza da tastiera in uno stepper lineare. Traccia lo stato aria-disabled del pulsante dell’intestazione dello step; il lavoro è completato quando, dopo aver completato il primo step e aver spostato il focus sull’intestazione successiva, la tecnologia assistiva non segnala più quel pulsante come disabilitato.

Scritto dal modello di indicizzazione a partire dal testo della issue.