[docs] Remove TypeScript v3 outdated version mentions

[oliviertassinari]

Steps to reproduce

We have a couple of mentions of TypeScript v3 https://github.com/search?q=org%3Amui+%22TypeScript+3.x%22&type=code. But per our version policy, following DefinitelyTypes we should remove them, we don't support this anymore see mui/material-ui#40996

Context

First raised in #14115 (comment).

Your environment

npx @mui/envinfo

  Don't forget to mention which browser you used.
  Output from `npx @mui/envinfo` goes here.

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[oliviertassinari]

@samuelsycamore maybe you want to take this one on, docs and DX related.

[github-actions]

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