markdown: Use buffer font instead of UI font for code blocks (#17351)

Related to #15379 (it does not fix the issue for inline code blocks)

#### Before

<img width="905" alt="Screenshot 2024-09-05 at 11 25 50 AM"
src="https://github.com/user-attachments/assets/38ac7b1a-1556-4b69-a74a-b0fca35d598c">

#### After

<img width="871" alt="Screenshot 2024-09-05 at 11 24 33 AM"
src="https://github.com/user-attachments/assets/a70c2624-c000-4b07-9fb2-940adf8e287f">


Release Notes:

- Updated Markdown code blocks to use the buffer font.

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>