Summary:
`FLAGS_ysql_yb_major_version_upgrade_compatibility` can be reset during the monitoring phase. New function `IsYsqlMajorVersionUpgradeInProgress` handles this case and will return true for the entire duration of the YSQL major (PG15) upgrade.

Fixes #26412
Jira: DB-15774

Test Plan: Jenkins

Reviewers: pjain, telgersma

Reviewed By: telgersma

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D42583