Create dedicated dev branch for active development

[linkdotnet]

Follow-Up to #333

Currently, master is the active development branch. That could mean that we already use previews of the .net SDK itself leading to unexpected behavior for folks that just want to try out the blog.

While the current approach is to use tags (https://github.com/linkdotnet/Blog/releases) for that, it is non-obvious. Therefore in the future master should be the stable branch and we create a new dev or unstable branch for development.

As master already is containing preview stuff, which I don't want to revert, we have to wait until .net 9 is stable to do the cut.