github: Update API rate limit exceeded detection in GitHubSession class

GitHub API seems to have harmonized its responses when API rate limit is
exceeded regardless an user is authenticated or not: a 403 status code is
returned with an error message starting with "API rate limit exceeded"
(https://docs.github.com/en/rest/overview/resources-in-the-rest-api
?apiVersion=2022-11-28#exceeding-the-rate-limit).

Previously, a 429 status code was returned for authenticated users.

So update code detecting exceeded rate limit accordingly and keep the
429 status check for backward compatibility just in case.

Fix #4429