GHSA-978j-88f3-p5j3: Threshold value is ignored (all shares are n=3)

Affected versions of this crate did not properly calculate secret shares requirements.

This reduces the security of the algorithm by restricting the crate to always
using a threshold value of three, rather than a configurable limit.

The flaw was corrected by correctly configuring the threshold.

References

• Nebulosus/shamir#3
• https://rustsec.org/advisories/RUSTSEC-2020-0160.html