Bump cimg/node to 18.7

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
cimg/node	docker	major	14.20 -> 18.7

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[mui-bot]

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:book:	Netlify deploy preview: https://deploy-preview-undefined--material-ui.netlify.app/

No bundle size changes

Generated by :no_entry_sign: dangerJS against 4be63177e16c01bfc8b82d3c2e1c846a971401f9

[michaldudak]

We remain on Node 14 for now