Deprecate `merge_trains_count` and `merge_train_car_index` on merge request GraphQL type

Problem to solve

The following discussion from !158832 (merged) should be addressed:

• @vwolanyk started a discussion: (+1 comment)

  Refactored to memoize this value as it is now used in resolver methods in this type.

  In thinking about the other 2 fields that are visible only when merge trains enabled - I am wondering if it might make more sense to consolidate all merge train related fields into one type:

  • move merge_trains_count
  • mobe merge_train_car_index(do we even need this here as it is an attr of merge train car) to MergeTrainCarType

  My rationale is that neither mergeTrainsCount OR mergeTrainIndex can exist if the MR does has not been added to a merge train

  @hfyngvason what do you think?

Proposal

1. Move :merge_train_car_index and :merge_trains_count to MergeTrainCarType
2. Deprecate the existing :merge_train_car_index and :merge_trains_count on MergeRequestType (see Graphql Deprecation and Removal Process for more info)