Add index to improve contribution graph query

What does this MR do and why?

Follow-up to !74970 (merged) to create the index synchronously and remove the old index.

The first migration is a no-op on GitLab.com because it has already been created over the weekend.

Migration output

== 20211206073851 CreateCalendarEventsIndexSynchronously: migrating ===========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:events, [:author_id, :project_id, :action, :target_type, :created_at], {:name=>"index_events_author_id_project_id_action_target_type_created_at", :algorithm=>:concurrently})
   -> 0.0147s
-- Index not created because it already exists (this may be due to an aborted migration or similar): table_name: events, column_name: [:author_id, :project_id, :action, :target_type, :created_at]
== 20211206073851 CreateCalendarEventsIndexSynchronously: migrated (0.0174s) ==

== 20211206074547 RemoveOldCalendarEventsIndex: migrating =====================
-- transaction_open?()
   -> 0.0000s
-- indexes(:events)
   -> 0.0065s
-- current_schema()
   -> 0.0003s
== 20211206074547 RemoveOldCalendarEventsIndex: migrated (0.0112s) ============

== 20211206074547 RemoveOldCalendarEventsIndex: reverting =====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:events, [:author_id, :project_id], {:name=>"index_events_on_author_id_and_project_id", :algorithm=>:concurrently})
   -> 0.0080s
-- add_index(:events, [:author_id, :project_id], {:name=>"index_events_on_author_id_and_project_id", :algorithm=>:concurrently})
   -> 0.0177s
== 20211206074547 RemoveOldCalendarEventsIndex: reverted (0.0289s) ============

== 20211206073851 CreateCalendarEventsIndexSynchronously: reverting ===========
-- transaction_open?()
   -> 0.0000s
-- indexes(:events)
   -> 0.0080s
-- remove_index(:events, {:algorithm=>:concurrently, :name=>"index_events_author_id_project_id_action_target_type_created_at"})
   -> 0.0090s
== 20211206073851 CreateCalendarEventsIndexSynchronously: reverted (0.0277s) ==

MR acceptance checklist

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