Use Sawyer Resource array notation in Github Importer

What does this MR do and why?

This is the third of a series of merge requests to convert Octokit returned objects into a hash before using it in the codebase in order to rely less on Sawyer::Resource objects

This change updates several classes in GitHub Import to use array notation instead of dot notation when accessing properties of Sawyer::Resource objects. Bear in mind that this change doesn't convert Octokit returned objects into hashes yet, which means that GitHub Import is still using Sawyer::Resource objects. A future MR will handle the conversion to a hash after all classes are converted to the array notation.

By the way, a few remaining classes still need to be converted, which will be handled in follow-up MRs.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/372472

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Create a new project, and choose to import from Github
2. Provide your personal access token
3. Then you should see a list of repositories
4. Select one and import it
5. Check if everything is imported

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.