add a note on max_prepared_stmt_count for 6.5.2

[ran-huang]

What is changed, added or deleted? (Required)

this note is missing from v6.5.2 because the pull request pingcap/tidb#43054 was merged after the code freeze.

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

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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 (in Chinese).

• master (the latest development version)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qiancai

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[ran-huang]

/verify

[ran-huang]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 73a1796

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13820.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #13821.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13822.