setuptools 68.1.0: Moving content to `[tool.setuptools]`

[akkornel]

Setuptools 68.1.0 moved [tool.setuptools] out of beta. Once we rely on setuptools 68.1.0 or later, we can start using it!

[akkornel]

Debian trixie has setuptools 78.1.1; when that becomes oldstable (in a couple years from now), we can action this!