WIP: tidb: update tidb_index_serial_scan_concurrency docs

[0xPoe]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

See: pingcap/tidb#64023

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from:
• 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-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign lilin90 for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Copilot]

Pull Request Overview

This PR updates documentation to reflect the deprecation of the tidb_index_serial_scan_concurrency variable. The variable is no longer modifiable, and its functionality has been replaced by tidb_executor_concurrency for sequential index scans and tidb_analyze_distsql_scan_concurrency for ANALYZE operations.

Key Changes:

• Deprecates tidb_index_serial_scan_concurrency with a warning that directs users to the replacement variables
• Removes references to the deprecated variable from troubleshooting and performance benchmarking guides
• Updates configuration examples to use tidb_analyze_distsql_scan_concurrency instead of the deprecated variables

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
system-variables.md	Adds deprecation warning and updates description to indicate the variable is kept for backward compatibility only
statistics.md	Consolidates documentation by removing the separate section for tidb_index_serial_scan_concurrency and updating tidb_analyze_distsql_scan_concurrency to note its expanded role
troubleshoot-tidb-oom.md	Removes deprecated variable from the list of concurrency controls for reducing memory usage
tidb-cloud/v8.5-performance-benchmarking-with-tpcc.md	Replaces deprecated variables with tidb_analyze_distsql_scan_concurrency in TPC-C test setup
tidb-cloud/v8.1-performance-benchmarking-with-tpcc.md	Replaces deprecated variables with tidb_analyze_distsql_scan_concurrency in TPC-C test setup
best-practices/tidb-best-practices.md	Updates reference from deprecated variable to tidb_executor_concurrency for controlling index scan behavior

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}