tikv-config-file: update the default value of use-unified-pool (#5537)

[ti-srebot]

cherry-pick #5537 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In docs-cn repo:
git pr https://github.com/pingcap/docs-cn/pull/5538

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/docs-cn.git pr/5538:release-5.0-1fd887326d25

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First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Update the default value of use-unified-pool from false to true.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: tikv-config-file: update the default value of use-unified-pool docs#4707
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@Joyinqin you're already a collaborator in bot's repo.