Update mandatory training and include list of roles

Why is this change being made?

This MR is in preparation for rolling security training out across GitLab:

• it creates a SSoT for the roles required to complete secure code training and a process for changing that
  • It may be that this gets extracted out somewhere else, in which case this MR (or the page itself after a merge) should be updated to point to it.
• it details what course must be completed, and by when
• it states what the expected result is (i.e. completion, not a certain score or lack of hints, etc. It's not an exam).

When can this be merged?

Once IT have enabled access to SCW via Okta for all the roles in this MR.

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, and the content is SAFE
• 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
  • The when to get approval handbook section explains the workflow in more detail
• 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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