release-8.5: update default value of fix control 52869

[time-and-fate]

What is changed, added or deleted? (Required)

Update the default value description of optimizer fix control 52869 to clarify that it is ON starting from TiDB v8.5.7.

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:
• Other reference link(s): planner: enable fix control 52869 by default to let planner consider IndexMerge when an alternative index exists | tidb-test=pr/2753 tidb#68920

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

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