Revert "Upgrade to toml v0.5.11" (#2058)
This _did_ fix https://github.com/charliermarsh/ruff/issues/1894, but was a little premature. `toml` doesn't actually depend on `toml-edit` yet, and `v0.5.11` was mostly about deprecations AFAICT. So upgrading might solve that issue, but could introduce other incompatibilities, and I'd like to minimize churn. I expect that `toml` will have a new release soon, so we can revert this revert.
Reverts charliermarsh/ruff#2040.
