[TextField] Accessibility contrast issue with filled variant

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The current filled variant of TextField does not pass the Lighthouse accessibility check on contrast.

Expected Behavior 🤔

The text color should follow the material design specs, which will make the Lighthouse accessibility check on contrast pass.

Steps to Reproduce 🕹

Steps:

1. Launch Chrome/Chromium
2. Go to https://next.material-ui.com/components/text-fields/
3. Launch Developer Tools
4. Run a Lighthouse accessibility audit

Context 🔦

I want filled variants of TextField to be accessible out of the box.

Your Environment 🌎

  System:
    OS: Linux 5.10 Fedora 33 (Workstation Edition) 33 (Workstation Edition)
  Binaries:
    Node: 14.15.4 - /usr/bin/node
    Yarn: Not Found
    npm: 6.14.10 - /usr/bin/npm
  Browsers:
    Chrome: Not Found
    Firefox: 85.0.1