Change fuzz test maturity to viable

Why is this change being made?

This MR updates the fuzz testing maturity from Minimal to Viable. Both coverage-guided and API fuzz testing are now at the point where they can be used in the real-world and deliver value to end-users. They have also been used at GitLab internally and were able to successfully find several issues. As documented in the CMS results, we have met the user test requirements for moving the maturity as well.

Required CMS score: 3.14

Actual CMS results:

Full CMS results: gitlab-org/ux-research#1225 (closed)

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
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