Pin and bump `bls12_381_plus` dependency

[wulfraem]

Description of change

The most recent version of bls12_381_plus introduces adds changes to type conversion that may break builds due to conflicting implementations of From trait.

error[E0119]: conflicting implementations of trait `From<bls12_381_plus::scalar::Scalar>` for type `Issuer`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/identity_credential-1.3.0/src/credential/issuer.rs:43:1
   |
43 | / impl<T> From<T> for Issuer
44 | | where
45 | |   T: Into<Url>,
   | |_______________^
   |
   = note: conflicting implementation in crate `bls12_381_plus`:
           - impl From<bls12_381_plus::scalar::Scalar> for <bls12_381_plus::scalar::Scalar as ark_ff::fields::prime::PrimeField>::BigInt;

Pinning this dependency to the previous version hotfixes build issues when used as an indirect dependency here (via zkryptium).

Version should be able to opened up in the future, but for now we should stick to the 0.8.15.

Type of change

Add an x to the boxes that are relevant to your changes.

• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] Enhancement (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Fix

How the change has been tested

cargo clean, deleting the Cargo.lock file, and running cargo build on the main branch produces the error described above. Doing the same on the PR's branch completes the build successfully.

Change checklist

Add an x to the boxes that are relevant to your changes.

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes