Add billing category to Fulfillment group

The Fulfillment group needs a billing category created in order to properly assign a feature category to a new Sidekiq worker related to Billing. This new feature category is pretty basic and will not be shown on marketing pages.

This MR creates the new category, Billing, and assigns it to the fulfillment group in the Growth stage.

Approvals

Merge requests with changes to stages and groups and significant changes to categories need to be created, approved, and/or merged by each of the below:

• EVP of Product: @sfwgitlab
• VP of Product Strategy @markpundsack
• The Product Director relevant to the stage group(s): @hilaqu
• The Engineering Director relevant to the stage group(s): @bmarnane
• CEO: @sytses

The following people need to be on the merge request so they stay informed:

• Senior Director of Development @clefelhocz1
• Director of Quality @meks
• The Product Marketing Manager relevant to the stage group(s)