tidb: update tidb_index_serial_scan_concurrency docs (#21938)

[ti-chi-bot]

This is an automated cherry-pick of #21938

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[gemini-code-assist]

Code Review

This pull request updates the documentation to reflect the deprecation of the tidb_index_serial_scan_concurrency system variable and clarifies the behavior of other ANALYZE concurrency variables. The review feedback points out critical issues, such as unresolved git merge conflict markers in system-variables.md, and suggests minor improvements for terminology capitalization, formatting consistency in SQL blocks, and maintaining a formal tone.

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[gemini-code-assist]

There are unresolved git merge conflict markers (<<<<<<< HEAD, =======, >>>>>>>) in this section. Please resolve the conflict by choosing the updated description and ensuring proper punctuation.

Suggested change

	<<<<<<< HEAD
	- This variable is used to set the concurrency of executing the automatic update of statistics.
	- Starting from v8.5.7, the default value of this variable changes from `1` to `2`. If your cluster is upgraded from an earlier version, the value of this variable remains unchanged after the upgrade.
	=======
	- This variable controls the concurrency for building statistics during auto `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously
	- Starting from v8.5.7 and v9.0.0, the default value of this variable changes from `1` to `2`. If your cluster is upgraded from an earlier version, the value of this variable remains unchanged after the upgrade.
	>>>>>>> 3d8180e56a (tidb: update tidb_index_serial_scan_concurrency docs (#21938))
	- This variable controls the concurrency for building statistics during auto `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously.
	- Starting from v8.5.7 and v9.0.0, the default value of this variable changes from `1` to `2`. If your cluster is upgraded from an earlier version, the value of this variable remains unchanged after the upgrade.

References

1. Avoid unnecessary words and repetition, and ensure correct English grammar, spelling, and punctuation. ^(link)