Migration to add root_namespace_id to project_statistics

What does this MR do and why?

Adds new column root_namespace_id to project_statistics table

Ref: #424728 (closed)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Database Migration

Up

ci: == [advisory_lock_connection] object_id: 343100, pg_backend_pid: 69750
main: == [advisory_lock_connection] object_id: 344140, pg_backend_pid: 69753
main: == 20230911095016 AddRootNamespaceIdToProjectStatistics: migrating ============
main: -- column_exists?(:project_statistics, :root_namespace_id)
main:    -> 0.0022s
main: -- add_column(:project_statistics, :root_namespace_id, :bigint)
main:    -> 0.0005s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE project_statistics ADD CONSTRAINT fk_198ad46fdc FOREIGN KEY (root_namespace_id) REFERENCES namespaces (id) ON DELETE SET NULL NOT VALID;")
main:    -> 0.0017s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- execute("ALTER TABLE project_statistics VALIDATE CONSTRAINT fk_198ad46fdc;")
main:    -> 0.0050s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0005s
main: -- index_exists?(:project_statistics, :root_namespace_id, {:name=>"index_project_statistics_on_root_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0038s
main: -- add_index(:project_statistics, :root_namespace_id, {:name=>"index_project_statistics_on_root_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0009s
main: == 20230911095016 AddRootNamespaceIdToProjectStatistics: migrated (0.0328s) ===

main: == [advisory_lock_connection] object_id: 344140, pg_backend_pid: 69753

Down

main: == [advisory_lock_connection] object_id: 226120, pg_backend_pid: 71785
main: == 20230911095016 AddRootNamespaceIdToProjectStatistics: reverting ============
main: -- remove_column(:project_statistics, :root_namespace_id, :bigint)
main:    -> 0.0018s
main: == 20230911095016 AddRootNamespaceIdToProjectStatistics: reverted (0.0081s) ===

main: == [advisory_lock_connection] object_id: 226120, pg_backend_pid: 71785

MR acceptance checklist

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