Update Fulfillment engineering workflow

Why is this change being made?

This is the first iteration on https://gitlab.com/gitlab-org/fulfillment-meta/-/issues/81

to update the workflow and avoid having 100% of the issues being a ~P1 and reduce stress towards the end of the milestone.

We'll use the Deliverable label to flag those issues that we want in the milestone, eliminating the need of priorities.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what