hard-coded glibc version in pythoin wheel build

[doug-leith]

In:

https://github.com/tensorflow/federated/blob/main/tensorflow_federated/tools/python_package/build_python_package.sh

the glibc version is hard-coded to 2.31 in the option "--plat-name=manylinux_2_31_x86_64". that results in the built wheel not being installable on machines with older versions of glibc. the dependency is not essential - tensorflow-federated compiles and runs just fine with glibc 2.28 for example.

suggested fix: there's no need for this to be hard-wired, it can be extracted on the fly from 'ldd --version' or the like.