[docs] Update README.md from alpha -> beta when beta is out

[neuhaus]

The software seems to be under active development, yet the main README file contains this outdated sentence [ed by tobi: "We foresee entering beta around the beginning of 2024."] that makes it appear as if the project hasn't been updated in a while.

Someone who is in the know about the roadmap should update it. Thanks 🙇🏼‍♂️