Downgrade SQL Server AOAG failover logs from error to warn

[bosconi]

Summary

• Downgrade tracing::error!("found uninitialized LSN!") to tracing::warn! in src/sql-server-util/src/cdc.rs
• Replace mz_ore::soft_panic_or_log!("upstream SQL Server went backwards in time...") with tracing::warn! in src/storage/src/source/sql_server/progress.rs

During SQL Server Always On Availability Group (AOAG) failovers, these two conditions can occur and are expected/recoverable, but were previously logged at error level — triggering unnecessary Sentry alerts. Both code paths already handle the conditions gracefully (skip and continue).

Fixes MaterializeInc/database-issues#9986

Test plan

• cargo check -p mz-sql-server-util -p mz-storage compiles cleanly
• Verify Sentry alerts stop firing for AOAG failover events after deployment
• Existing SQL Server source tests continue to pass (no behavioral change — only log level changed)

https://claude.ai/code/session_015rRQ5puqtBufPEQ5RoUtjt

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