Create foreign key constraints in seperate migrations

To comply with the style guides at Gitlab.

db/migrate/20200825084819_create_security_findings_table.rb

@@ -8,9 +8,9 @@ class CreateSecurityFindingsTable < ActiveRecord::Migration[6.0]
   disable_ddl_transaction!
 
   def up
-    create_table :security_findings, id: :bigserial do |t|
-      t.references :scan, null: false, foreign_key: { to_table: :security_scans, on_delete: :cascade }
-      t.references :scanner, null: false, foreign_key: { to_table: :vulnerability_scanners, on_delete: :cascade }
+    create_table :security_findings, if_not_exists: true do |t|
+      t.references :scan, null: false
+      t.references :scanner, null: false
       t.integer :severity, limit: 2, index: true, null: false
       t.integer :confidence, limit: 2, index: true, null: false
       t.text :project_fingerprint, index: true, null: false

db/migrate/20200828155134_add_foreign_key_on_scan_id_to_security_scans.rb

+# frozen_string_literal: true
+
+class AddForeignKeyOnScanIdToSecurityScans < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_foreign_key :security_findings, :security_scans, column: :scan_id, on_delete: :cascade # rubocop:disable Migration/AddConcurrentForeignKey
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_foreign_key :security_findings, column: :scan_id
+    end
+  end
+end

db/migrate/20200828155205_add_foreign_key_on_scanner_id_to_vulnerability_scanners.rb

+# frozen_string_literal: true
+
+class AddForeignKeyOnScannerIdToVulnerabilityScanners < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_foreign_key :security_findings, :vulnerability_scanners, column: :scanner_id, on_delete: :cascade # rubocop:disable Migration/AddConcurrentForeignKey
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_foreign_key :security_findings, column: :scanner_id
+    end
+  end
+end

db/schema_migrations/20200828155134

+b3ee994231a8da694dbcda227b37e19a2112be666648d918425b064ec19d239e
\ No newline at end of file

db/schema_migrations/20200828155205

+b575558752206149171a05231e4167e1ac3e1295f76d800edfe3d61c1b996b52
\ No newline at end of file