Initialize conversion of ci_builds_metadata.id for bigint migration

What does this MR do?

Related to #325616 (closed)

Migration up:

== 20210705142714 InitializeConversionOfCiBuildsMetadataIdToBigint: migrating =
-- table_exists?(:ci_builds_metadata)
   -> 0.0011s
-- column_exists?(:ci_builds_metadata, :id)
   -> 0.0052s
-- column_exists?(:ci_builds_metadata, :id)
   -> 0.0011s
-- columns(:ci_builds_metadata)
   -> 0.0010s
-- add_column(:ci_builds_metadata, "id_convert_to_bigint", :bigint, {:default=>0, :null=>false})
   -> 0.0025s
== 20210705142714 InitializeConversionOfCiBuildsMetadataIdToBigint: migrated (0.0253s) 

== 20210705143150 BackfillCiBuildsMetadataIdForBigintConversion: migrating ====
-- table_exists?(:ci_builds_metadata)
   -> 0.0012s
-- column_exists?(:ci_builds_metadata, :id)
   -> 0.0014s
-- column_exists?(:ci_builds_metadata, :id)
   -> 0.0011s
-- column_exists?(:ci_builds_metadata, "id_convert_to_bigint")
   -> 0.0013s
unknown OID 28: failed to recognize type of 'relfrozenxid'. It will be treated as String.
unknown OID 1034: failed to recognize type of 'relacl'. It will be treated as String.
unknown OID 194: failed to recognize type of 'relpartbound'. It will be treated as String.
== 20210705143150 BackfillCiBuildsMetadataIdForBigintConversion: migrated (0.0468s) 

Migration down:

== 20210705143150 BackfillCiBuildsMetadataIdForBigintConversion: reverting ====
-- execute("DELETE FROM batched_background_migrations WHERE job_class_name = 'CopyColumnUsingBackgroundMigrationJob' AND table_name = 'ci_builds_metadata' AND column_name = 'id' AND job_arguments = '[[\"id\"],[\"id_convert_to_bigint\"]]'")
   -> 0.0048s
== 20210705143150 BackfillCiBuildsMetadataIdForBigintConversion: reverted (0.0051s) 

== 20210705142714 InitializeConversionOfCiBuildsMetadataIdToBigint: reverting =
-- remove_column(:ci_builds_metadata, "id_convert_to_bigint")
   -> 0.0009s
== 20210705142714 InitializeConversionOfCiBuildsMetadataIdToBigint: reverted (0.0046s) 

Does this MR meet the acceptance criteria?

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