Allow sodium_compat v1.6+ or v2.x

[IanSimpson]

This is instead of https://github.com/pusher/pusher-http-php/pull/385 which was submitted 4 months ago, but not accepted after review.

This retains the minimum of 1.6, but also allow 2.x

More info: https://paragonie.com/blog/2024/04/release-sodium-compat-v2-and-future-our-polyfill-libraries

[JamesFreeman]

Any updates on this @fbenevides?

[BenSeDev]

I've opened an issue for this (#392), to keep track on this. Referencing this PR. I've forked this PR and tried it out. Everything looks ok. So 🤞 for a quick merge.