Bump Ruby version to 2.7.6

CI has been using 2.7.6 since gitlab-build-images!518 (merged). GDK is being updated in gitlab-development-kit!2781 (merged).

Related to https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/7243

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.