docs: use a shorter SHOW VARIABLES example (#21794)

[ti-chi-bot]

This is an automated cherry-pick of #21794

What is changed, added or deleted? (Required)

• replace the outdated SHOW GLOBAL VARIABLES LIKE 'tidb%' example with a shorter verified SHOW GLOBAL VARIABLES LIKE 'tidb_stmt_summary%' example
• avoid the disputed tidb_wait_split_region_finish row in the example output
• keep the other SQL examples unchanged

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: docs: use a shorter SHOW VARIABLES example docs#23177
• Other reference link(s):
  • Verified against a local TiDB v8.5.6 instance for the replacement example query.

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

[qiancai]

/approve

[ti-chi-bot]

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