Replace TI missing weight policy with processor

What does this MR do and why?

Threat insight currently has a policy in place to put an issue back into refinement state if it is set ready for development without a weight.

We would like to change this behaviour to just tag the actor who changed the state to ready for development to let them know they missed assigning a weight.

This MR is set for the sandbox project only, I will follow up with an MR to remove the existing policy and enable the processor for gitlab.org if testing goes OK.

Expected impact & dry-runs

• Add a new issue on the https://gitlab.com/gitlab-org/quality/engineering-productivity/triage-ops-playground project
• All the labels groupthreat insights typefeature backend and workflowready for development

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.