Fix MarlinPC's CommitterKey to return the correct supported_degree

[Will-Lin4]

Description

Fixes MarlinPC's CommitterKey supported_degree function to return the correct degree from the number of powers. The number of elements in powers is equal to supported_degree + 1, as specified in the trim function here:

poly-commit/src/marlin/marlin_pc/mod.rs

Line 96 in 9698a9b

let powers = pp.powers_of_g[..=supported_degree].to_vec();

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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.
• [NA] Wrote unit tests (Existing unit tests use supported_degree already to generate polynomials)
• [NA] Updated relevant documentation in the code (Existing documentation is sufficient)
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer