perf: KZG verification uses `JointScalarMultiplication`

[yelhousni]

Description

Replace two ScalarMultiplication by a JointScalarMultiplication in KZG verification.

Type of change

• [x] New feature (non-breaking change which adds functionality)

How has this been tested?

kzg package tests pass.

How has this been benchmarked?

saves ~1%

Checklist:

• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] 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] I did not modify files generated from templates
• [x] golangci-lint does not output errors locally
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules