Improve convergence plot: larger fonts, serif numbers, remove annotations

- Increased font sizes significantly (axis labels: 24, tick labels: 20, legend: 18)
- Increased figure size to (20, 10) for better readability without zooming
- Set tick labels to use serif font (DejaVu Serif or fallback)
- Removed all step info box annotations
- Removed title
- Made y-axis labels black
- Matched style to right plot visualizations

Author: serboba