Drop support for python3.8

What does this MR do and why?

• Drop support for python3.8

We need to drop support for python3.8 in order to be able to upgrade to cython3. See in particular !159 (comment 1619762810) That MR depends on this one.

Changes per file:

.gitlab-ci.yml:

• remove build targets for python3.8 from .gitlab-ci.yml
• replace python3.8 with python3.11 in linux-debug target (not entirely clear if this makes sense, it's used in test/coverage, which only runs on git.ligo.org).

environment*.yml:

• no longer need to restrict networkx to be <3

setup.cfg:

• update python requires to >=3.9
• add 3.11 as one of the programming languages
• no longer need to restrict networkx to be <3

requirements.rst

• update the requirement for the minimal python version.

How to set up and validate locally

1. check that the pipeline only runs for 3.9 and higher: all builds should succeed now.
2. for checking the changes in setup.cfg:
   1. locally create a minimal conda environment with 3.8:

      conda create -n test python=3.8
      conda activate test

   2. install dependencies:
      conda install suitesparse 'cython<3' numpy scipy
   3. try to install finesse using pip:
      pip install .
      This should give the error
      ERROR: Package 'finesse' requires a different Python: 3.8.18 not in '>=3.9'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Includes tests for added/modified code
• Added or modified the documentation if relevant
• Docstrings added/modified for new/modified functions
• Committer/reviewer has ran the documentation stage when ready to merge