Follow-up from "Fix duplicated events on VSM stage form"

The following discussion from !29030 (merged) should be addressed:

• @engwan started a discussion: (+2 comments)

  Hmm.. can you help me understand why we have duplicate keys here?

  Like why do we need IssueFirstMentionedInCommit => 6 in EE, if we already have IssueFirstMentionedInCommit => 2 in CE?

Fix:

1. Remove the IssueFirstMentionedInCommit from EE
2. Create a DB migration that updates the stage identifiers in the database (6 -> 2)
3. Remove the uniq call (https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/analytics/cycle_analytics/stage_events.rb#L151).