Fix issue email participants migration version

What does this MR do?

This MR fixes the version number of a migration so it runs before the post migration counterpart. This should make the execution safe when the self-hosted instance runs the pre and post migrations at once.

More info: !48711 (comment 510227176)

Migration

The index already exists in PRD, we just fix the version number.

== 20201128210000 AddServiceDeskReplyToIsNotNullIndexOnIssuesFix: migrating ===
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, [:id], {:name=>"idx_on_issues_where_service_desk_reply_to_is_not_null", :where=>"service_desk_reply_to IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0099s
== 20201128210000 AddServiceDeskReplyToIsNotNullIndexOnIssuesFix: migrated (0.0103s)

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