onnxruntime

Remove Python 3.7 from the python packaging pipeline (#14887)

### Description
1. Remove Python 3.7 from the python packaging pipeline. It is planned
for the next release and approved by the PMs. Also we will add 3.11, but
it will be addressed in another PR.
2. Stop generating python packages based on Ubuntu 18.04 which will
reach EOL next month. We will either replace them with Ubuntu 20.04 or a
CentOS 8 variant.