Fix Inconsistent index_design_management_versions index

What does this MR do and why?

The index index_design_management_versions_on_author_id was added in this MR. For some, unknown reason we could not find it in our production database.

I am adding the index back to avoid inconsistency between our Gitlab.com and self-host databases.

up:

== 20220116175851 AddAuthorIndexToDesignManagementVersions: migrating =========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:design_management_versions, :author_id, {:where=>"author_id IS NOT NULL", :name=>"index_design_management_versions_on_author_id", :algorithm=>:concurrently})
   -> 0.0065s
-- Index not created because it already exists (this may be due to an aborted migration or similar): table_name: design_management_versions, column_name: author_id
== 20220116175851 AddAuthorIndexToDesignManagementVersions: migrated (0.0098s)

down:

== 20220116175851 AddAuthorIndexToDesignManagementVersions: reverting =========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:design_management_versions, :author_id, {:name=>"index_design_management_versions_on_author_id", :algorithm=>:concurrently})
   -> 0.0036s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- remove_index(:design_management_versions, {:name=>"index_design_management_versions_on_author_id", :algorithm=>:concurrently, :column=>:author_id})
   -> 0.0042s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20220116175851 AddAuthorIndexToDesignManagementVersions: reverted (0.0105s)

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Related to #349549 (closed)