[CHORE] Bump .nvmrc to Node v18.18.0.

[1Copenut]

Summary

PR follows #7274 that bumps Docker to use node-18.18.0-slim to set Node v18.18.0 as our daily driver for EUI.

QA

Remove or strikethrough items that do not apply to your PR.

General checklist

QA is a two-part manual process.

Part I
Pull the branch locally, then enter the following commands on your terminal:

• nvm install 18.18.0
• yarn you may see a Node warning about fsevents optional dependency error. This is b/c of our older Jest dep.
• yarn lint
• yarn test-unit
• yarn test-cypress

Part II
When all of the following tests complete successfully:

• nvm alias default 18.18.0
• nvm uninstall 18.17.1

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7276/
• Storybook: https://eui.elastic.co/pr_7276/storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 2dba31a

cc @1Copenut

[cee-chen]

Can we just merge/cherry-pick this into #7274? I'd rather not local CI and fall out of sync with each other, and I don't see a reason to split these up into separate PRs

[1Copenut]

Closing this PR on @cee-chen suggestion to fold it into #7274.