fix: Replaced Bookerly with Noto Serif for licensing reasons (#1736)

## Summary

Fixes #258. Bookerly is not licensed for use in CrossPoint. Switched to
Google's open Noto Serif font.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

scripts/generate_kerning_ligature_epub.py

@@ -149,15 +149,15 @@ except ImportError:
 
 
 _PROJECT_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-_BOOKERLY_FONT = os.path.join(
+_NOTOSERIF_FONT = os.path.join(
     _PROJECT_ROOT, "lib", "EpdFont", "builtinFonts", "source",
-    "Bookerly", "Bookerly-Regular.ttf",
+    "NotoSerif", "NotoSerif-Regular.ttf",
 )
 
 
 def _get_font(size=20):
-    """Get the Bookerly font at the requested size, with system fallbacks."""
-    paths = [_BOOKERLY_FONT]
+    """Get the NotoSerif font at the requested size, with system fallbacks."""
+    paths = [_NOTOSERIF_FONT]
     for path in paths:
         try:
             return ImageFont.truetype(path, size)