Rename MECC to CORE - GitLab's Management Philosophy

This merge request updates the name MECC (Managing so Everyone Can Contribute) to CORE: GitLab's Management Philosophy.

This does not impact the name The Workplace OS, which will remain the umbrella term that captures a set of modules (e.g. All-Remote, CORE, and other modules to-be-defined) which support the modern workplace.

Why do this?

• MECC was a first iteration and served us well in getting GitLab's management philosophy documented
• CORE is catchier and more memorable, which is useful to upcoming goals of driving certifications of the philosophy
• CORE is an acronym. As we've seen with CREDIT, this makes a key part of GitLab easier to remember, recall, and implement in day-to-day work
• GitLab is open core built entirely with open source. CORE is a nod to open core.

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.

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