Re-name VSA StageSummary class to match convention

The following discussion from !29791 (merged) should be addressed:

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

  I'm curious as to why this doesn't follow the pattern Gitlab::Analytics::CycleAnalytics::Summary::Group::StageSummary.

  As in - why isn't it Gitlab::Analytics::CycleAnalytics::Summary::Group::StageTimeSummary?

  (not blocking, just curious)