Update `tox-monorepo` for tox 4.0, update CI to consume tox 4.0

[scbedd]

Tox 4.0 is a fairly massive shift in internals even if the migration guide doesn't seem too bad.

We shipped tox-monorepo for tox<4. This enables us to re-use the same tox.ini file regardless of where we're starting from. You just need to provide the relative path to the tox.ini we use.

This issue covers support for tox>=4, and all the changes that are necessary to enable that.

• git+https installs need to shift to new method instead of old egg method.
• Any transitive dependencies that need to be updated due to the tox update.

[kdestin]

Resolved by #30159