fix: resolve node-gyp / node-sass errors on build

[mcataford]

Description

It looks like node-sass crashed on build, which prevented going forward with some changes. Since the minimum Node version currently in maintenance is v12, this sets the minimum / preferred versions to that via nvm. node-sass is also bumped to the newest eligible version, which seems to solve the build issues.

The Node version used in CI is also changed to match what we'd expect via nvm.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

[github-actions[bot]]

:tada: This PR is included in version 8.0.5 :tada:

The release is available on:

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

Your semantic-release bot :package::rocket: