Migrate config builder_url to builder_urls (#1662)

sequencer/api/migrations/V34__builder_urls.sql

@@ -0,0 +1,9 @@
+-- When multi-builder support was added, the configuration field `builder_url: Url` was replaced by
+-- an array `builder_urls: Vec<Url>`. If the saved config has no `builder_urls` field, it is older
+-- than this change. Populate `builder_urls` with a singleton array formed from the old value of
+-- `builder_url`, and delete the no longer used `builder_url`.
+UPDATE network_config
+   SET config =
+        jsonb_set(config, '{config,builder_urls}', jsonb_build_array(config->'config'->>'builder_url'))
+        #- '{config,builder_url}'
+ WHERE NOT (config->'config' ? 'builder_urls');