[core] Update minimum version of Node.js to 14.0.0

[DiegoAndai]

@oliviertassinari's edits: the rationale:

• The ecosystem largely moved from using a more recent version of Node.js: [v6] Upgrade minimum version of Node.js #32546 (comment). So no need to carry legacy code around for <5% of the community. However, Supporting Node.js v14 has no clear cost, so no need to degrade the DX for a good % of the community by bumping to a larger version.
• Sync with MUI X: [core] Update minimum supported version of Node.js to 14.0.0 mui-x#6966

[mui-bot]

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

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Generated by 🚫 dangerJS against 3d82bf0

[DiegoAndai]

Changed this PR to upgrade node, syncing it with MUI X for a smoother experience while using both.

[oliviertassinari]

👌

[Janpot]

@DiegoAndai Just noticed, @mui/material has it at 12 still, as well as in the docs it's at 12. Opening a PR: #43315.

[oliviertassinari]

I guess we should update those as well?

material-ui/.browserslistrc

Line 78 in 87aad6d

node 12.0

[aarongarciah]

@oliviertassinari updating .browserslist in #43326