Add working-by-default to product principles.

Why is this change being made?

We have seen time and again that feature adoption lags when the feature is not enabled and working by default. This MR is an important and meaningful tweak to the on-by-default principle. IT is not sufficient for the feature to be on-by-default, it needs to work. We think this will

• Spur adoption of features
• Give us better feedback and reduce cycle time
• Provide more value to our users
• Reduce time to value for our users
• Reduce complexity and improve usability of using GitLab

This comes at the cost of additional complexity for the R&D organization. We will have to figure out the right working setup and we will risk escalations when we get these wrong.

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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