Simply buildMembershipTypeValues & access cached values

Simply buildMembershipTypeValues & access cached values

This simplifies this function & switches it to use a cached function to get the MembershipTypes. In general I would
argue that we should be caching metadata entities (membership types, statues, price fields, etc) and accessing
the cached versions & ideally the v4 api would be able to access cached versions filter-dependent.

In order to keep the test passing I worked a little on standardisation