Replace rand/rand_core with ark_std::rand

[weikengchen]

Description

This PR replaces the use of rand or rand_core to ark_std::rand.

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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md --- N/A, does not affect a developer.
• Re-reviewed Files changed in the Github PR explorer