master: correct stats variable defaults

[qiancai]

What is changed, added or deleted? (Required)

Correct the documented default values for stats-related system variables:

• tidb_auto_analyze_concurrency: 1 -> 3 starting from v8.5.7
• tidb_auto_build_stats_concurrency: 1 -> 2 starting from v8.5.7
• tidb_sysproc_scan_concurrency: 1 -> 4 starting from v8.5.7

Also update the statistics.md auto-analyze variable table so it is consistent with system-variables.md, and mention the upgrade behavior in the variable descriptions.

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

• 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)

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

• This PR is translated from: master: correct stats variable defaults docs#23115 N/A
• Other reference link(s):
  • Related v8.5 PR: v8.5.7: correct stats variable defaults for v8.5 docs#23114

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

[github-actions]

Auto-sync completed successfully

Source PR: pingcap/docs#23115
Target PR: #21731

Chinese documentation has been updated based on English documentation changes.

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