Summary:
FLAGS_TEST_ysql_block_writes_to_catalog is added to simulate the behavior of DDLs during YSQL major upgrade.

Fixes #27185
Jira: DB-16671

Test Plan: Jenkins

Reviewers: dsherstobitov, telgersma

Reviewed By: dsherstobitov, telgersma

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D43970