[DataGrid] First class support of v5

[oliviertassinari]

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

Summary 🔦

A list of tasks to do in order to drop @material-ui/core v4 and only support v5 going forward.

Details

• Wait for v5 to have enough adoption -> 1/3 relative to v4 could be a great threshold to prioritize this effort.
• Update docs, createMuiTheme->createTheme [DataGrid] Drop support of v4 below 4.12.0 #2281
• Migrate the docs and dev tooling to v5 two weeks before the stable release of v5. [docs] Use same infra from v5 #2441
• Remove getDoc prop. See [TrapFocus] Capture nodeToRestore via relatedTarget material-ui#26696 for why. It solves our cross-document issue at the same time. This is wrong ⬇️

https://github.com/mui-org/material-ui-x/blob/9dd5cf94061d1733566b7b66982fae554ebcf242/packages/grid/_modules_/grid/components/panel/GridPanelWrapper.tsx#L28

First step in #2224

• Remove duplicated modules, for instance: https://github.com/mui-org/material-ui-x/blob/a4df486a95b72847224b71291fcdc3211feea3d1/packages/grid/_modules_/grid/utils/material-ui-utils.ts#L7
• Remove conditional code. [core] Remove v4 conditional code #2575
  https://github.com/mui-org/material-ui-x/blob/1a0ad36f7380ba6bff04fa8f2b6366e001988734/packages/grid/_modules_/grid/components/GridPagination.tsx#L107
• Revert 36124c6?
• Rename imports from @material-ui to @mui. [core] Support for @mui packages #2571

[embeddedt]

Any chance that e1d572f will land in a v4-compatible release?

[oliviertassinari]

@embeddedt Developers should be able to use MUI X v5 (the next version we work on, relying on MUI Core v5) inside an application that is built on @material-ui/core v4. Developers will need to re-configure the theme (for v4 and v5 core) and (disable the global class names of v4). Would this be enough?

[embeddedt]

Yes; that would be perfect!

[DanailH]

Created #2990 to track the effort on fixing the inline previews on the DataGrid pages for v5.
I'm closing this issue since this is the last item on the list and it is not blocking for v5 stable release.