Support new Pandoc 3.4 behavior with GFM math

[cderv]

closes #2572

As explained in this issue, Pandoc does by default uses the alternative GFM syntax for math.

In github_document(), we choose to keep writing the dollars syntax $$ and $ to avoid any future breakage.

In the future, we may add an option to activate the writing of ```math and $` if necessary. It seems they were added by Github to handle specific characters
https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions

You can either surround the expression with dollar symbols ($), or start the expression with $and end it with$. The latter syntax is useful when the expression you are writing contains characters that overlap with markdown syntax.

[yihui]

Sounds good to me. Thanks!