chore(.nvmrc): specify `v20` instead of `lts/*`

[adrianschmidt]

This ensures that when revisiting an old branch in the future, the project will still build successfully with the specified Node.js version, even if the latest LTS version at that time is incompatible.

Review:

• [ ] Commits are atomic
• [ ] Commits have the correct type for the changes made
• [ ] Commits with breaking changes are marked as such

Browsers tested:

(Check any that applies, it's ok to leave boxes unchecked if testing something didn't seem relevant.)

Windows:

• [ ] Chrome
• [ ] Edge
• [ ] Firefox

Linux:

• [ ] Chrome
• [ ] Firefox

macOS:

• [ ] Chrome
• [ ] Firefox
• [ ] Safari

Mobile:

• [ ] Chrome on Android
• [ ] iOS

[github-actions[bot]]

Documentation has been published to https://lundalogik.github.io/lime-elements/versions/PR-2867/

[lime-opensource]

:tada: This PR is included in version 37.13.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: