[tserver] Add YSQL enabled check to UpgradeYsql and corresponding test (#27167)

[ok-Mook]

This PR addresses issue #27167.

• Added an early exit check in TabletServiceAdminImpl::UpgradeYsql to handle the case when YSQL is not enabled.
• Added a corresponding integration test in YsqlMajorUpgradeTest to verify correct behavior when YSQL is disabled.
• Unable to fully verify locally due to macOS build environment conflicts, but submitting for alternative validation.

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[iSignal]

@ok-Mook : Thanks for raising this PR. Looks like this was recently addressed by @hari90 in a similar change but feel free to pick up any other interesting issues, for ex: #27168 that was opened by @hari90 recently.