config: disable witness related schedulers by default

[rleungx]

What problem does this PR solve?

Issue Number: close #8891.

What is changed and how does it work?

Since it's not GA, I prefer to disable it by default.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Connor1996
• lhy1024

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[Connor1996]

LGTM

[codecov]

Codecov Report

Merging #7765 (6119f44) into master (0d10882) will decrease coverage by 0.29%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head 6119f44 differs from pull request most recent head faa4b07. Consider uploading reports for the commit faa4b07 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7765      +/-   ##
==========================================
- Coverage   69.99%   69.71%   -0.29%     
==========================================
  Files         430      430              
  Lines       47632    47632              
==========================================
- Hits        33340    33205     -135     
- Misses      11540    11670     +130     
- Partials     2752     2757       +5     

Flag	Coverage Δ
unittests	69.71% <ø> (-0.29%)	⬇️

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[nolouch]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 6119f44

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8892.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #8893.