[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 🌎

`npx @material-ui/envinfo`

  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

[oliviertassinari]

@Dripcoding You can go ahead 😃

[oliviertassinari]

It looks like you’re trying to change something else here.

@eps1lon I see 4 values:

1. Update the palette to match the spec
2. Bring AA contrast on the filled input
3. Fix the ugly grey background-color
4. Remove the potential confusion around the filled input being disabled

I think that 1. can be solved independently. I don’t think that 2,3,4 can.