i18n(ko-KR): add v6.mdx translation

[nurooong]

Description (required)

• i18n(ko-KR): add v6.mdx translation
• Translation of src/content/docs/en/guides/upgrade-to/v6.mdx

Related issues & labels (optional)

• Suggested label: i18n

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Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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