executor,util: write slow query to slow log no matter what log level (#30461)

[ti-srebot]

cherry-pick #30461 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/30494

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---

What problem does this PR solve?

Issue Number: close #30309

Problem Summary:

What is changed and how it works?

The slow log uses the default log level that is info level.
Executor writes slow query by log.WARN methods

Check List

Tests

• Unit test
• Integration test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox
• youjiali1995

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[ti-srebot]

/run-all-tests

[winoros]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 80bf3a5

[ti-chi-bot]

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/2ead84d9572000991047ccc9a6df9a2971608488