Pinning Cryptography Dependency

[phutelmyer]

Recent versions of cryptography have shown failures in the rust requirements / installations. Pinning for now.

Describe the change
Updating backend and mmrpc requirements with a pinned cryptography.

Describe testing procedures
Built Strelka locally and within Github Actions

Sample output
N/A
Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings