logutil: force level of slow logger to "debug"

[SabaPing]

What problem does this PR solve?

Fix oncall issue https://internal.pingcap.net/jira/browse/ONCALL-3746.

Setting TiDB's log.level to error causes that the slow log does not print.
One must set the log.level to info or warn via tiup cluster edit-config and reload the TiDB node to re-enable the slow logs.

This PR fix this issue. After fixing, global log.level should not affects slow logs.

Manual test steps

Start TiDB with command line arguement -L=error.

Slow logs should output normally.

Release note

Fix an error that cause slow log missing when the log level is set to `error` or `fatal`.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breeswish
• crazycs520

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[crazycs520]

LGTM

[breezewish]

Please fill the release note, and pick up suitable release versions.

[breezewish]

@SabaPing PTAL

[SabaPing]

@SabaPing PTAL

I will back to this issue late afternoon today.

[breezewish]

@SabaPing Do you have updates~?

[SabaPing]

@SabaPing Do you have updates~?

Ahhh, I spent more time than expected on development yesterday, and I also handled a few oncalls on cloud in the afternoon, so I really didn't have time.

I hope I can handle this issue this afternoon.

[SabaPing]

Manual test steps:

1. Start TiDB with command line arguement -L=error.
2. Slow logs should output normally.

[SabaPing]

Fixed in #30461.