Make glfw-redist dependency a floating dependency

[NogginBops]

Description

https://github.com/opentk/glfw-redist is a package that quite often experiences issues or needs to be updated for some reason.
And these fixes are not automatically updated for users of the OpenTK nuget package.

We should change it so that OpenTK has a floating dependency on glfw-redist so that it's always possible to push new fixes of that package and have them apply to basically all OpenTK users immediately without having to make a new OpenTK release.

[NogginBops]

There doesn't look like there is any way to get the floating version described here to work with paket.
This is the page the describes how paket dependencies and their versions can be specified: https://fsprojects.github.io/Paket/nuget-dependencies.html#Version-constraints

I'm thinking this might need to be postponed until we move away from paket towards a more "vanilla" build system.

[NogginBops]

Moving this to 4.8.0 as there doesn't seem to be a fix available for 4.7.3.

[NogginBops]

Still can't find any good solutions using paket. Postponing this further.
At least we've updated the glfw version in relation to this PR #1574