bug(form-field): Prefix and suffix text color mismatched from spec

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

Currently the text appears black instead of the gray color in the spec.

### Reproduction

Spec: https://m2.material.io/components/text-fields
Prefix and suffix demo: https://material.angular.io/components/form-field/examples#form-field-prefix-suffix.

### Expected Behavior

Text is gray

### Actual Behavior

Text is black

### Environment

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