Update configured Ruby version to 2.6.6

Why is this change being made?

Ensures the Ruby people use to build locally matches the CI/CD pipeline, which was recently upgraded to 2.6.6 in: gitlab-org/gitlab-build-images!329 (merged).

Also adds a .tool-versions file for the first time, which is for folks using asdf instead of rvm or rbenv: https://asdf-vm.com/#/core-configuration.

Similar to:

• https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/.tool-versions
• https://gitlab.com/gitlab-org/gitlab-pages/-/blob/master/.tool-versions

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.