Upgrade to Ruby 2.6.5

Why is this change being made?

Issue: #6389 (closed)

Main GitLab repository depends on Ruby 2.6.5, but www-gitlab-com still uses 2.6.3.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what