harmonize the roadmap priorities with service templates

We are using "Must have" instead of "Need to have" there, and it seems
like a more natural wording that we should also use here. It also
happens to reuse RFC-like vocabulary so it may be more explicit for
some people.

roadmap/2021.md

@@ -21,7 +21,7 @@ attention. In general, the over-arching goals are therefore:
    service retirements)
  * better communication (particularly with developers)
 
-## Need to have
+## Must have
 
  * [ ] email delivery improvements:
    * [x] handle bounces in CiviCRM ([issue 33037](https://gitlab.torproject.org/tpo/tpa/team/-/issues/33037))
@@ -207,7 +207,7 @@ but it should still be an interesting exercise to perform.
 
 The following is a review of the [2020 roadmap](roadmap/2020).
 
-## Need to have
+## Must have
 
  * [x] retiring old machines (moly in particular)
  * [x] move critical services in ganeti