[CI] Drop manylinux2014 wheels in 3.1 version

[jakirkham]

Previously a GLIBC check was added in PR: #11289

This has caused Conda users some issues: #11348

Recently conda-forge added a patch to disable the GLIBC check in PR: conda-forge/xgboost-feedstock#240

Wondering if there is a better way to address this issue to eliminate the need for a patch

[hcho3]

Easy way: stop shipping manylinux2014 wheels on PyPI. The glibc check exists only to provide a deprecation warning for manylinux2014. Once we remove the warning, we can remove the glibc check too.

[trivialfis]

Easy way: stop shipping manylinux2014 wheels on PyPI.

Sounds good to me.

[hcho3]

Let's drop manylinux2014 in the 3.1 release.