Change broadcast message index

What does this MR do?

Adds the correct index for broadcast messages after a discussion.

Related to #38341 (closed)

I added 6000 broadcast messages (since indexes only get used after a certain threshold) to my local database to test if the index gets used, see the following results for before (sequence scan) and after (index scan).

Before

pry(main)> BroadcastMessage.where('ends_at > :now', now: Time.current).order_id_asc.explain
  BroadcastMessage Load (11.5ms)  SELECT "broadcast_messages".* FROM "broadcast_messages" WHERE (ends_at > '2020-01-29 15:28:07.750183') ORDER BY "broadcast_messages"."id" ASC
=> EXPLAIN for: SELECT "broadcast_messages".* FROM "broadcast_messages" WHERE (ends_at > '2020-01-29 15:28:07.750183') ORDER BY "broadcast_messages"."id" ASC
                                      QUERY PLAN
---------------------------------------------------------------------------------------
 Sort  (cost=120.13..120.99 rows=341 width=686)
   Sort Key: id
   ->  Seq Scan on broadcast_messages  (cost=0.00..105.79 rows=341 width=686)
         Filter: (ends_at > '2020-01-29 15:28:07.750183'::timestamp without time zone)
(4 rows)

After

pry(main)> BroadcastMessage.where('ends_at > :now', now: Time.current).order_id_asc.explain
  BroadcastMessage Load (88.6ms)  SELECT "broadcast_messages".* FROM "broadcast_messages" WHERE (ends_at > '2020-01-29 15:24:56.880528') ORDER BY "broadcast_messages"."id" ASC
=> EXPLAIN for: SELECT "broadcast_messages".* FROM "broadcast_messages" WHERE (ends_at > '2020-01-29 15:24:56.880528') ORDER BY "broadcast_messages"."id" ASC
                                               QUERY PLAN
---------------------------------------------------------------------------------------------------------
 Index Scan using broadcast_messages_pkey on broadcast_messages  (cost=0.28..214.10 rows=5818 width=591)
   Filter: (ends_at > '2020-01-29 15:24:56.880528'::timestamp without time zone)
(2 rows)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

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