v6.3: add sm4 for tiflash to encryption-at-rest.md

[lidezhu]

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• This PR is translated from: v6.3: add sm4 for tiflash to encryption-at-rest.md docs-cn#11237
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Oreoxmt
• ran-huang

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[Oreoxmt]

LGTM

[ran-huang]

/merge

[ti-chi-bot]

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

Commit hash: 8d24bec