source-brevo: update nltk dependency

`airbyte-cdk` versions 6.6.3 is the earliest version that allows `nltk` version 3.9.1, so `source-brevo` needed updated to at least that version of `airbyte-cdk`.

`source-brevo`'s snapshot tests pass, so the connector still runs. But I didn't do any local stack testing - we don't have any test credentials plus there are no active `source-brevo` captures in production, so I didn't think this was worth the time.

Author: Alex-Bair