tiflash: correct the default value of task_scheduler_active_set_soft_limit (#20474)

[ti-chi-bot]

This is an automated cherry-pick of #20474

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[ti-chi-bot]

@hfxsd This PR has conflicts, I have hold it.
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@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	17ea9af	link	true	/test pull-verify

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