Modified alembic migration to downgrade properly

Previously, the downgrade was hitting a violation of the old unique
constraint on leaderboard_scores if multiple versions of scores were
stored for the same address.

The modified downgrade only retains the score for each address on the
latest publich version of each leaderboard.

engineapi/alembic/versions/cc80e886e153_added_leaderboard_versions_table_and_.py

@@ -105,6 +105,23 @@ def upgrade():
 
 
 def downgrade():
+    op.execute(
+        """
+        WITH latest_version_for_leaderboard AS (
+            SELECT leaderboard_id, MAX(version_number) AS latest_version
+            FROM leaderboard_versions WHERE published = true
+            GROUP BY leaderboard_id
+        )
+        DELETE FROM leaderboard_scores WHERE
+            (leaderboard_id, leaderboard_version_number) NOT IN (
+                SELECT
+                    leaderboard_id,
+                    latest_version AS leaderboard_version_number
+                FROM
+                    latest_version_for_leaderboard
+            )
+        """
+    )
     # ### commands auto generated by Alembic - please adjust! ###
     op.drop_constraint(
         op.f("fk_leaderboard_scores_leaderboard_id_leaderboard_versions"),