Deprecate toolchains hub repo

[aignas]

It was fixed in #3001 where this comment/idea was floated: https://github.com/bazel-contrib/rules_python/pull/3001#issuecomment-2981231509

The ideal way would be to ask users to use toolchain lookup instead but we need to ensure that they know how to use that.

[rickeylev]

Thanks for filing this.

Middle ground/transition idea: replace those aliases (which use select() and duplicate all the logic the toolchains have) with a wrapper that applies a python version transition and does a toolchain lookup. I wonder if that transition being applied might actually fix some issues? (e.g. --python_version=X.Y, but someone using a @python_X_Z reference)

This eliminates the problematic part (the duplicated select-branching conditions) and should be cheap to carry forward for an indeterminate time.