Fix milestone title font problem (#22863)
Replace #22853 since it's closed, and actually there are 2 places need
to be fixed.
~~Follow @fsologureng 's suggestion to keep the `<hX>` tags.~~
Update: from fsologureng: this doesn't change anything from a11y's point
of view. So I think this PR could be fine to fix the UI looking problems
as a quick patch, then defer the a11y problems to new PRs together.
Before: the font-size is too large.
After: it seems better.
![image](https://user-images.githubusercontent.com/2114189/218266257-fc2d5872-9e96-4c6a-87ea-f27531ac15c0.png)
![image](https://user-images.githubusercontent.com/2114189/218266247-efc09d83-405f-4495-967a-30d9744134ce.png)
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>