[YSQL] Change some backfill log levels

[jasonyb]

Jira Link: DB-18380

Description

Logs of the form

Backfilling indexes { REDACTED } for tablet REDACTED from key 'REDACTED' (task=0x0000329bf8ed9d58, state=kRunning): TS REDACTED: Backfill Index Table RPC failed for tablet REDACTED: Remote error (yb/rpc/outbound_call.cc:565): Service unavailable (yb/tserver/tablet_service.cc:742): Not enough time left 130.953s (rpc error 4)

are WARNING level but are completely expected, so they should be changed to INFO level.

Logs of the form

Backfilling indexes { REDACTED } for tablet REDACTED from key '' (task=0x00001157fd02fb18, state=kRunning): Scheduling retry with a delay of 1037ms (attempt = 7 / 150)...

are INFO level but are bad for performance as they indicate that a master-tserver BackfillChunk RPC did not successfully report its progress by the deadline, so that progress is wasted and retried. So these should be made WARNING level.

See also #28584.

Issue Type

kind/enhancement

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