Remove cfssl certificate issuance instructions (#2962)

[ti-chi-bot]

This is an automated cherry-pick of #2962

Updated both English and Chinese guides to only describe certificate issuance using cert-manager for TiDB clusters. Removed all steps and references to cfssl, simplifying the documentation and focusing on the recommended cert-manager approach.

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• master (the latest development version for v1.x)
• feature/v2 (the latest development version for v2.x)
• v2.0 (TiDB Operator 2.0 versions)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)

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[hfxsd]

/approve

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