Rename "sub-values" to operating principles

As part of the MECC build-out and the June 2022 e-group offsite, we are renaming sub-values to operating principles.

The context/agenda item that led to this is as follows: MECC and its examples are heavily values-based. Is there anything else we can do to reinforce values in a visible way and link them to management philosophy?

The goal is for sub-values to be actionable, and to guide behavior. Being clear about their function as an operating principle allows them to be more easily lived by GitLab team members, and less esoteric to other organizations who seek to learn from GitLab's Remote Playbook and MECC management philosophy.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • 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 affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---