Pin `thredded` to recent Git commit

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In #609, we had to use an older version of html-parser (see commit 456ed14) for the forum engine thredded to still work with Rails 7.1. This was necessary due to https://github.com/thredded/thredded/issues/979. In the meantime, glebm made a commit on thredded to deal with this internally in their project, so we don't need to include the dependency html-parser on our side (see https://github.com/thredded/thredded/pull/981). Therefore, we get rid of html-parser and use the new commit from their GitHub repo directly.

For reviwers

• Run bundle install locally.
• Additionally, you will have to run docker compose build mampf webpacker, otherwise bundler is complaining in the Docker images.
  If you happen to know a way to avoid rebuilding the whole image if we just update some gems let me know since rebuilding always takes quite some time and I'd like to avoid that. I'm also wondering why this is even the case after having run bundle install only locally; why is Bundler complaining in the Docker container after that? I thought it has its own dependencies installed inside the docker container...