Update dependencies across the project

[rduffr]

There are many libraries in this package that are outdated and are creating dependency resolution issues for projects. Two key ones are markup and http. I pulled down the code and updated locally and the only compile time issues arose in the project's "markdown.dart" file due to breaking changes in markdown 4.0.0. I made the changes and things appear to work but I have not fully tested them. The dependencies should be updated in this project.

[nico-famedly]

If you have a PR with the code changes to upgrade the markdown package, that would be very appreciated. There are 2 open PRs, that update the package without changing the usage code, which isn't really mergable :D

[rduffr]

I submitted a PR with changes.

[vanyasem]

related #1563