[Grid2] Rollout plan

[oliviertassinari]

Summary

We recently made Grid v2 stable, #43054 so now it's time to look at the rollout plan, effectively to get completely get rid of the old one.

A possible action plan:

• 1. Update the h1 in https://next.mui.com/material-ui/react-grid/ to mention deprecated, so Google and Algolia search results are clear
• 2. ?Reexport Grid v2 from @mui/system/Grid2 getting one step closer to move everyone imports from @mui/material to mui/system [RFC] Remove @mui/material default theme and @mui/system, @pigment-css/react reexports #40594
• 3. Migrate all the MUI codebase to the Grid v2
• 4. Introduce a deprecation warning when using Grid v1
• 5. In the next major swap the implementation for the Grid namespace, put the deprecated Grid v1 into the exported GridLegacy namespace and deprecate Grid2 imports
• 6. In the major after that next major, remove Grid2 and GridLegacy.

Motivation

Simplicity

[DiegoAndai]

The plan looks good overall, @oliviertassinari 👌🏼

The only step I'm not sure of is:

2. Reexport Grid v2 from @mui/system/Grid2 getting one step closer to move everyone imports from @mui/material to mui/system [RFC] Remove @mui/material default theme and @mui/system, @pigment-css/react reexports #40594

I don't know the long-term vision for the system package, and we need to solve that before setting the goal of moving everyone to import Grid from it. This is not the issue to discuss this. @aarongarciah and @colmtuite should probably lead the discussion.