Update the remark-gfm dep

[Billos]

Is your feature request related to a problem? Please describe. From Markdown v2.5.2, you can use

[link](URL){:target="_blank"}

Which would be nice to have in the web UI so that we don't leave the app when we click on a markdown link

Describe the solution you'd like I think a simple solution would be to update the remark-gfm dependency, it's currently to 1.0.0, and latest version is 4.0.0

[jmattheis]

Will be updated in https://github.com/gotify/server/pull/696