Downgrade to Ruby 2.6.6
After upgrade to Ruby 2.7.2 (!1225 (merged)), the link checker for documentation started intermittently failing. Retries seem to always fix it:
- Failed job: https://gitlab.com/gitlab-org/gitlab/-/jobs/791090474.
- Successful retry: https://gitlab.com/gitlab-org/gitlab/-/jobs/791482837.
It is relatively easy to reproduce locally - do a full rebuild a couple of times and check the links.
It may be related to: https://github.com/nanoc/nanoc/issues/1501
Until fixed, we'll downgrade to Ruby 2.6.6 for stability. This MR allows the project to work well inside a Ruby 2.6.6 environment. The next step is to downgrade the environment itself.
CC @gl-docsteam