project.toml version and released versions appear to be out of sync

[kleinschmidt]

There have been two PRs that have bumped the version in Project.toml:

• compat fixes for Julia v1.8 #459 bumped to 1.6.1 (compat fixes)
• Implement fstatistic #424 bumped to 1.6.0 (added feature)

but as far as I can tell both the registry and the github release are stuck at 1.5.1. Given that we never released 1.6.1, we should revert the version to 1.6.0 and tag a release (I'd selfishly like to wait until #462 lands which is a non-breaking deprecation that improves API compatibility and does not require a minor bump IMO).