Fix Code Review JTBD

Why is this change being made?

Updates the JTBD listed in https://about.gitlab.com/direction/create/#jobs-to-be-done.

• Fix broken source links
• Add missing secondary JTBD from gitlab-org/ux-research#941
• Remove maturity grade
• Clarify confidence level

Review app:

• Code Review: https://pedroms-master-patch-72943.about.gitlab-review.app/direction/create/#jobs-to-be-done
• Secure: https://pedroms-master-patch-72943.about.gitlab-review.app/handbook/engineering/ux/stage-group-ux-strategy/sec/jtbd/#job-statements

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 one of the people listed in the "Maintained by" section in 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 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. (this requirement has been removed pending identification of a new DRI for the handbook)