Update PG submodule to 2.20

Bump PG from PG-2.19 (88420fc9) to PG-2.20 (0c7f42dc).

Major changes in PG 2.20 (632 commits):
- plots.pl: new display-only 2D graph macro (JSXGraph + pgfplots)
- Extensive POD restructuring across all macro categories
- Translation updates for 12+ languages
- plotly3D parameter parsing fix
- Formula string parenthesization improvements
- JSXGraph and npm dependency updates
- Sample problem fixes (TriangleGraphTool, AnswerWithUnits)

Docker build and render confirmed working (simple numeric,
DropDown, MathJax all verified).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: drdrew42