outdate docs about privilege requirements for the target database

[july2993]

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Feature Request

Is your feature request related to a problem? Please describe:

https://docs.pingcap.com/tidb/stable/tidb-lightning-faq

admin user privileges in the downstream TiDB no longer need

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Optimization: