Eliminate duplicated VSA event

What does this MR do?

This MR fixes a small inconsistency in Value Stream Stage event values in the codebase. The change is not visible to the end user.

The Issue first mentioned in commit event presents in both FOSS and EE with different identifier (number)

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/analytics/cycle_analytics/stage_events.rb#L13
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/analytics/cycle_analytics/stage_events.rb#L18

This caused a bug where duplicated events showed up in the dropdown (fixed): !29030 (merged)

The fix was simply calling uniq on the events, however it didn't fix the underlying problem.

This MR:

1. Remove the event definition from EE (unlikely that it was ever used due to the uniq call)
2. Just to be safe, create a data migration where existing records are fixed.

Migration

up

== 20210119122354 AlterVsaIssueFirstMentionedInCommitValue: migrating =========
== 20210119122354 AlterVsaIssueFirstMentionedInCommitValue: migrated (0.0096s)

down

NOOP

Update query

Plan

Simple EachBatch on the primary key. Updating 100 rows at once (0 affected records on PRD).

UPDATE "analytics_cycle_analytics_group_stages" SET "start_event_identifier" = 2 WHERE "analytics_cycle_analytics_group_stages"."id" >= 394 AND "analytics_cycle_analytics_group_stages"."id" < 540 AND "analytics_cycle_analytics_group_stages"."start_event_identifier" = 6 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #214631 (closed)