Use alternative mechanism(s) to upgrade shillelagh

[turkoid]

shillelagh.functions.upgrade uses pip to upgrade the module. This is a hidden dependency that is not automatically installed.

If using a different package manager such as uv I have to add pip as a requirement for my project, which doesn't seem right.

Either remove the upgrade function all together, or use a different mechanism to upgrade within the tool.

[betodealmeida]

Removing sounds a bit harsh, since it's a useful feature and pip is probably still the most popular way of installing Python packages.

Ideally we'd improve the function to check for pip/uv, and then have it do the right thing.