WIP: add v6.5.12 release notes

[qiancai]

What is changed, added or deleted? (Required)

Add v6.5.12 release notes

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)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 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)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from: WIP: add v6.5.12 release notes docs-cn#19853
• Other reference link(s):

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• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[qiancai]

Suggested change

- (dup): release-7.5.5.md > Compatibility changes - Add a new system variable [`tidb_ddl_reorg_max_write_speed`](https://docs.pingcap.com/tidb/v7.5/system-variables#tidb_ddl_reorg_max_write_speed-new-in-v755) to limit the maximum speed of the ingest phase when adding indexes [#57156](https://github.com/pingcap/tidb/issues/57156) @[CbcWestwolf](https://github.com/CbcWestwolf)

- (dup): release-7.5.5.md > Compatibility changes - Add a new system variable [`tidb_ddl_reorg_max_write_speed`](https://docs.pingcap.com/tidb/v6.5/system-variables#tidb_ddl_reorg_max_write_speed-new-in-v6512) to limit the maximum speed of the ingest phase when adding indexes [#57156](https://github.com/pingcap/tidb/issues/57156) @[CbcWestwolf](https://github.com/CbcWestwolf)