sql_server: Adjust sources logging levels to avoid sentry alerts

[martykulma]

Changes logging level for "restore detected" from ERROR to WARN to avoid creating sentry alerts.

Motivation

These 2 specifically came up recently, but I reviewed the logging in PG and MySQL (they seem ok):

DATABASE-BACKEND-41B
DATABASE-BACKEND-41C

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.