Set minimal python version to 3.11

pyFV3 requires a minimum python version 3.11. This was updated two
months ago. This commit cleans up the following two leftovers:

1. In `setup.py`, update the classifiers
2. In the `Dockerfile`, start from the python 3.11 image

Author: Roman Cattaneo

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.8
+FROM python:3.11
 
 RUN apt-get update &&\
     apt install -y --no-install-recommends \

setup.py

@@ -35,8 +35,7 @@
         "License :: OSI Approved :: BSD License",
         "Natural Language :: English",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.8",
-        "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.11",
     ],
     description="pyFV3 is a NDSL-based FV3 dynamical core for atmospheric models",
     install_requires=requirements,