Update remark-gfm to latest version

[alexcjohnson]

Currently we have a dependency on remark-gfm: ^3.0.1. There's a v4 but when we tried updating to that in #247 it broke the tests. Further investigation is needed.

Note that react-markdown is already limited to stay at major version 8, because major version 9 requires React 18 and Dash still needs to be compatible with React 16 (in fact 16 is still the default but even after we change the default to 18 we'll want to maintain compatibility with 16 for some time.)