Update to node.js v18

[jtsaito]

Depends on

• Fetch mock #20
• node18 supported for custom GitHub actions (Support node20 in runs for JavaScript actions actions/runner#2619)
• tested successfully on a repo
• version 1.2.0 tested an tagged

This enables

• Bump @octokit/graphql from 5.0.6 to 6.0.1 #15
• Bump @octokit/plugin-paginate-graphql from 2.0.3 to 3.0.0 #17

[jtsaito]

@jansiwy It seems we cannot do this update because GitHub actions does not support node.js 18 for custom actions yet

• When will be able to specify `runs.using: node18` in action.yml? actions/runner-images#7664
• Support node20 in runs for JavaScript actions actions/runner#2619

I think we should still roll out the refactored API calls and tests.

[jansiwy]

@jtsaito then please open a support tickets because it's kind of weird if GitHub has deprecated node.js 16 in octokit, but is not supporting node.js 18 in GitHub Actions.

[jtsaito]

I'm closing this because we have to wait for GitHub to support node.js 18 for GitHub actions.