Add new maintenance subtype labels to templates

As part of our team discussion this week, maintenance subtype labels are becoming more important to the company. Let's increase the odds of good data coming from the Technical Writing team by adding the appropriate subtype label to the two merge request templates we use in this repository.

(Noticed as part of creating !2999 (merged) on Russell's behalf. If we have typemaintenance as a default, we might as well set a default subtype too.)