cdc: add a description about checksum compatibility

[3AceShowHand]

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[Oreoxmt]

Suggested change

	升级集群时，需要先升级 TiCDC，后升级 TiDB。升级过程中，TiCDC 处于高版本，TiDB 处于低版本时，支持处理由低版本 TiDB 写入的 Checksum。升级完成后，应该保证 TiDB 和 TiCDC 使用相同的版本的 Checksum 校验算法。
	在升级集群时，需要首先升级 TiCDC，然后升级 TiDB。
	在升级过程中，如果 TiCDC 版本高于 TiDB 版本时，TiCDC 可以处理由较低版本 TiDB 写入的 Checksum 数据。
	升级完成后，为确保数据一致性，需要确保 TiDB 和 TiCDC 使用相同版本的 Checksum 校验算法。否则，可能会导致校验不一致或数据丢失的问题。

用户如何查看 TiDB 和 TiCDC 的 Checksum 校验算法版本，如果有对应的文档请添加对应链接

[3AceShowHand]

用户不需要关心 checksum 校验算法的版本，这是 tidb 和 ticdc 内部校验机制。具体对应的版本，在文中有提及。

[Oreoxmt]

升级完成后，应该保证 TiDB 和 TiCDC 使用相同的版本的 Checksum 校验算法。

@3AceShowHand 如果用户不需要关心，那这里“应该保证 TiDB 和 TiCDC 使用相同的版本的 Checksum 校验算法”主语是谁呢