Provide a default implementation of SSHUserPrivateKey#getPrivateKey

[Vlatombe]

This method is deprecated, and implementation still need to implement it. We have default methods now, so let's just leverage that to avoid useless boilerplate.

Testing done

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[Vlatombe]

Gentle ping to @jtnord or @olamy since you last merged PRs in this repository.

[jtnord]

given it was deprecated 11 years ago would it not behove us to remove it instead?

[Vlatombe]

@jtnord Feel free to do so, but considering the number of existing impls, I didn't want to bother breaking them.