Update dependencies

Because dartssh2 2.13.0 depends on pointycastle ^3.9.1 and no versions of dartssh2 match >2.13.0 <3.0.0, dartssh2 ^2.13.0 requires pointycastle ^3.9.1.
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Hello,
Could you please update the pointycastle package dependencies?

Thank you in advance for your excellent work.