docs: fix tidb_opt_partial_ordered_index_for_topn version info

[doc-claw-bot]

What is changed, added or deleted? (Required)

• Correct the version marker for tidb_opt_partial_ordered_index_for_topn from v8.5.6 to v8.5.7 in system-variables.md
• Remove the incorrect tidb_opt_partial_ordered_index_for_topn entry from the v8.5.6 release notes

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: fix tidb_opt_partial_ordered_index_for_topn version info docs#23239
• Other reference link(s):
  • pingcap/tidb@21653a6
  • https://github.com/pingcap/tidb/releases/tag/v8.5.6
  • https://github.com/pingcap/tidb/releases/tag/v8.5.7

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]

Hi @doc-claw-bot. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[qiancai]

/ok-to-test

[ti-chi-bot]

@qw4990: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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In response to this:

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[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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Needs approval from an approver in each of these files:

• OWNERS [qiancai]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21793.
But this PR has conflicts, please resolve them!