Unlimit xgboost version

[VesnaT]

What's your use case?

It seems that the latest xgboost MacOS package is missing libomp.dylib.
Therefore the xgboost version has been limited in #6857 to <2.1.

What's your proposed solution?

Unlimit the xgboost version when the issue is fixed on the xgboost side.

Are there any alternative solutions?
Instal libomp manually and move xgboost to optional requirements.

[VesnaT]

dmlc/xgboost#10448

[markotoplak]

Fortunatelly #6584 did not get merged yet so dropping a requirement is easy.

[jakirkham]

cc @hcho3 (for awareness)

[hcho3]

See dmlc/xgboost#10440 to learn more about the rationale behind dropping libomp.dylib from the wheel. In short, we probably won't restore libomp.dylib in the wheel in the foreseeable future, since we think having an ImportError with a readable error message is preferable to random crashes and hangs.

Given that the issue is only affecting users of Pip, can you relax the version pins for users of Conda?