Update to MonoGame v3.8.4.1

[RootCubed]

I noticed that external references that were using a newer MonoGame version would not be loaded correctly (in my case, LDtkMonogame) and thus no new files using that library would be added to the .mgcb file. Upping the version inside of Contentless fixes this.

[Ellpeck]

Hmm, that's sort of odd since the references to MonoGame are private and so they shouldn't really have an effect on what you're describing. But oh well, updates are good either way!

Thanks for this PR :)

[RootCubed]

I didn't quite confirm the root cause, but my best guess as to what was happening is that the library with the ContentPipeline I was using also imported MonoGame, but Contentless couldn't load the newer MonoGame version because the old one was already loaded (I saw some errors about it not being able to load MonoGame.Framework when I did some print debugging). But yeah, I also thought it was weird. Either way, thanks for merging!

[RootCubed]

Sorry to bother you again, but I think something went wrong during the NuGet upload, as when I downloaded the package, the tools folder seemed to still contain the old dependencies ^^

[Ellpeck]

Hm, that's odd. I'll try another release!