Drop support for Python 3.8 in GH action conda-packages.yaml workflow

Since NumPy 1.25 requires Python version >3.8, and with https://github.com/IntelPython/dpctl/pull/1282 and https://github.com/IntelPython/dpctl/issues/1295, it makes sense for us to stop building `numba-dpex` for 3.8.