Fix deadlock in drop_subscriptions_before_start (#10806)

ALTER SUBSCRIPTION requires AccessExclusive lock
which conflicts with iteration over pg_subscription when multiple
databases are present
and operations are applied concurrently.

Fix by explicitly locking pg_subscription
in the beginning of the transaction in each database.

## Problem
https://github.com/neondatabase/cloud/issues/24292

compute_tools/src/sql/drop_subscriptions.sql

@@ -2,6 +2,7 @@ DO $$
 DECLARE
     subname TEXT;
 BEGIN
+    LOCK TABLE pg_subscription IN ACCESS EXCLUSIVE MODE;
     FOR subname IN SELECT pg_subscription.subname FROM pg_subscription WHERE subdbid = (SELECT oid FROM pg_database WHERE datname = {datname_str}) LOOP
         EXECUTE format('ALTER SUBSCRIPTION %I DISABLE;', subname);
         EXECUTE format('ALTER SUBSCRIPTION %I SET (slot_name = NONE);', subname);