[material-ui][docs] Fix ESLint error in Stepper demo

[aarongarciah]

Part of #42548

Fixing an error reported by eslint-plugin-react-compiler in a Stepper demo:

/material-ui/docs/data/material/components/steppers/HorizontalNonLinearStepper.js
  51:5  error  This mutates a variable that React considers immutable  react-compiler/react-compiler

/material-ui/docs/data/material/components/steppers/HorizontalNonLinearStepper.tsx
  53:5  error  This mutates a variable that React considers immutable  react-compiler/react-compiler

How to test

• ESLint shouldn't report the error anymore (you need to set ENABLE_REACT_COMPILER_PLUGIN to true in .eslintrc.js locally)
• The demo should work as expected:
  • Original: https://mui.com/material-ui/react-stepper/#non-linear
  • PR: https://deploy-preview-42559--material-ui.netlify.app/material-ui/react-use-media-query/#migrating-from-withwidth

[mui-bot]

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https://deploy-preview-42559--material-ui.netlify.app/

Bundle size report

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[oliviertassinari]

Ah nice, this one indeed seemed like a bug.