Update variable names in historical reviewer tracking

The following discussion from !187457 (merged) should be addressed:

• @cindy-halim started a discussion:

  suggestion (non-blocking): thoughts on renaming these variables for more clarity?

          const eligibleReviewers = toUsernames(users);
          const unselectedEligibleReviewers = difference(eligibleReviewers, this.currentSelectedReviewers);
  
          setReviewersForList({
            issuableId: this.issuableId,
            listId: this.uniqueId,
            reviewers: unselectedEligibleReviewers,
          });
        }