Proposal: use latest middleman from Github instead of gem release

[tombye]

The middleman gem hasn't be released since Oct 2023 (see https://github.com/middleman/middleman/commits/main/CHANGELOG.md).

The community forums have also been pretty quiet for about the same time: https://forum.middlemanapp.com/.

But their dependabot has been happily updating dependencies on their main branch and there are still maintainers merging pull requests with fixes in and tidying up dead code: https://github.com/middleman/middleman/commits/main/.

Seeing as main is actually more of a representation of the middleman code working with the current state of the ruby ecosystem as it is at any given point, it seems like we should move to that instead of the gem.