Prepare async indexes for ci table int8 swaps

What does this MR do?

See #325617 (closed) and #325619 (closed)

Migration up

$ rails db:migrate:up VERSION=20210824174615
== 20210824174615 PrepareCiBuildsMetadataAndCiBuildAsyncIndexes: migrating ====
-- index_exists?(:ci_builds_metadata, :id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_metadata_on_id_convert_to_bigint, :algorithm=>:concurrently})
   -> 0.0030s
-- add_index_options(:ci_builds_metadata, :id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_metadata_on_id_convert_to_bigint, :algorithm=>:concurrently})
   -> 0.0000s
-- index_exists?(:ci_builds_metadata, :build_id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_metadata_on_build_id_convert_to_bigint, :algorithm=>:concurrently})
   -> 0.0020s
-- add_index_options(:ci_builds_metadata, :build_id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_metadata_on_build_id_convert_to_bigint, :algorithm=>:concurrently})
   -> 0.0000s
-- index_exists?(:ci_builds_metadata, :build_id_convert_to_bigint, {:where=>"has_exposed_artifacts IS TRUE", :name=>:index_ci_builds_metadata_on_build_id_int8_and_exposed_artifacts, :algorithm=>:concurrently})
   -> 0.0017s
-- add_index_options(:ci_builds_metadata, :build_id_convert_to_bigint, {:where=>"has_exposed_artifacts IS TRUE", :name=>:index_ci_builds_metadata_on_build_id_int8_and_exposed_artifacts, :algorithm=>:concurrently})
   -> 0.0000s
-- index_name_exists?(:ci_builds_metadata, :index_ci_builds_metadata_on_build_id_int8_where_interruptible)
   -> 0.0007s
-- index_exists?(:ci_builds, :id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_on_converted_id, :algorithm=>:concurrently})
   -> 0.0084s
-- add_index_options(:ci_builds, :id_convert_to_bigint, {:unique=>true, :name=>:index_ci_builds_on_converted_id, :algorithm=>:concurrently})
   -> 0.0000s
== 20210824174615 PrepareCiBuildsMetadataAndCiBuildAsyncIndexes: migrated (0.0578s)

Migration down

$ rails db:migrate:down VERSION=20210824174615
== 20210824174615 PrepareCiBuildsMetadataAndCiBuildAsyncIndexes: reverting ====
== 20210824174615 PrepareCiBuildsMetadataAndCiBuildAsyncIndexes: reverted (0.0186s)

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Does this MR meet the acceptance criteria?

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