Stop running CI jobs on Debian 8 (Jessie)

Looks like root SSL certificates shipped with Jessie have become stale.
Pip fails to interact with PyPI:
    $ ./.venv/bin/python -m pip install --upgrade "pip<19.2" "setuptools<44.0" "wheel<0.34"
    Cannot fetch index base URL https://pypi.python.org/simple/
    <...>
    No distributions at all found for pip<19.2 in ./.venv/lib/python3.4/site-packages

Jessie has reached EOL in 2018, Jessie LTS in 2020.
Even though Jessie ELTS is still supported, we will drop support now.

Author: sio

.github/workflows/test.yml

@@ -19,7 +19,6 @@ jobs:
       matrix:
         image:
           - centos-7
-          - debian-8
           - debian-9
           - debian-10
           - debian-11

tests/README.md

@@ -47,7 +47,6 @@ Automated tests are continuously executed after each push to this repo.
   infrastructure provided by [GitHub Actions]. The following configurations are
   being tested:
     - Centos 7: bash 4.2, sed 4.2
-    - Debian 8: bash 4.3, sed 4.2
     - Debian 9: bash 4.4, sed 4.4
     - Debian 10: bash 5.0, sed 4.7
     - Debian 11: bash 5.1, sed 4.7