Replace go-migrate with morph (#19116)

db/migrations/postgres/000042_create_threads.up.sql

@@ -0,0 +1,16 @@
+CREATE TABLE IF NOT EXISTS threads (
+    postid VARCHAR(26) PRIMARY KEY,
+    replycount bigint,
+    lastreplyat bigint,
+    participants text
+);
+
+ALTER TABLE threads ADD COLUMN IF NOT EXISTS channelid VARCHAR(26);
+
+CREATE INDEX IF NOT EXISTS idx_threads_channel_id ON threads (channelid);
+
+UPDATE threads
+    SET channelId=posts.channelid
+    FROM posts
+    WHERE posts.id=threads.postid
+    AND threads.channelid IS NULL;