Drop explicit TensorBoard dependency

[kmaziarz]

During integration with syntheseus, I found a conflict between the current dependency pins in RetroChimera and those of the MEGAN model: we have a strict pin of tensorboard==2.12.0, which is not compatible with tensorflow==2.20.0 pinned in MEGAN. However, RetroChimera itself doesn't use TensorBoard directly anymore: we used to use it for logging, but have since switched to Weights & Biases. It is still an indirect dependency through syntheseus-root-aligned -> OpenNMT-py -> tensorboard>=2.3; after dropping the strict pin in pyproject.toml, the dependency resolver can choose whatever works for MEGAN and OpenNMT (in this case, the newest 2.20.0 is fine).

Also, I raise the minimum version of syntheseus to 0.7.2, to pull in a recent checkpoint downloading bugfix from 0.7.2.