pkg/util: refine tidb_server_memory_limit to make the cpu usage more stable (#48927)

[ti-chi-bot]

This is an automated cherry-pick of #48927

What problem does this PR solve?

Issue Number: close #48741

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

Resolved a bug to improve CPU stability under prolonged high memory usage in tidb-server.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@ff2fcec). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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##             release-6.5     #49059   +/-   ##
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  Misses                 ?      75272           
  Partials               ?      16558           

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guo-shaoge, wshwsh12

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• OWNERS [guo-shaoge,wshwsh12]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-04 07:32:44.470718821 +0000 UTC m=+1426393.135945028: ☑️ agreed by wshwsh12.
• 2023-12-04 08:09:15.381810037 +0000 UTC m=+1428584.047036231: ☑️ agreed by guo-shaoge.

[hawkingrei]

/ok-to-test