ticdc: add scheduler.region-count-per-span in v8.5.4 release notes (#22688)

[ti-chi-bot]

This is an automated cherry-pick of #22688

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• Other reference link(s):ticdc: add scheduler configuration recommendations for table split mode #22483

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[hfxsd]

/approve

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[gemini-code-assist]

Code Review

This pull request updates the release notes for version 8.5.4 by adding a new configuration parameter, scheduler.region-count-per-span, for TiCDC. This parameter controls how tables are split into sub-tables based on Region counts during changefeed initialization in the new TiCDC architecture. Feedback was provided to rephrase the description of this parameter to use active voice and improve conciseness, aligning with the documentation style guide.