[docs] Add @mui/styles to list of packages that can have their versio…

[jiangdonghan]

During my upgrade from v5 to v6, I noticed that the documentation doesn't clearly address the status of @mui/styles. This package is marked as legacy (https://mui.com/system/styles/basics/) but its handling during the upgrade process isn't explicitly mentioned. Thanks!

• [ ] I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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• docs/data/material/migration/upgrade-to-v6/upgrade-to-v6.md

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