Define level of impact of updating default image configured for Docker executor on Hosted Runners for GitLab.com

We've been discussing several times about the need to update the default image we define for Hosted Runners' docker executor.

First, we use ruby:3.1 which is slightly outdated. Second, usage of the ruby:* image is driver basically by historical reasons and that's probably not the best choice. The image is quite heavy and pulling it into the job environment takes about 20 seconds (counted towards CI/CD minutes usage of the user) while a different image like ubuntu:latest is smaller (only 5 seconds for download) and should be enough to support the very basic jobs.

Depending on the default image is not ideal and probably a big part of our users (hopefully more than 90-95%) are defining custom images in their .gitlab-ci.yml files anyway. But before we will do any changes, we should first estimate how big impact such change would have.