Fix failed update with parallel workers

When executing "ALTER EXTENSION timescaledb UPDATE TO ..." it will fail
if parallel workers spawn for the update itself. Disable parallel
execution during the update.

sql/updates/pre-update.sql

@@ -5,6 +5,11 @@
 
 -- This file is always prepended to all upgrade scripts.
 
+-- Disable parallel execution for the duration of the update process.
+-- This avoids version mismatch errors that would have beeen triggered by the
+-- parallel workers in ts_extension_check_version().
+SET LOCAL max_parallel_workers = 0;
+
 -- Triggers should be disabled during upgrades to avoid having them
 -- invoke functions that might load an old version of the shared
 -- library before those functions have been updated.