Fixup: drop python 3.8 add 3.12 for conda on lassen

Author: ggeorgakoudis

.gitlab/jobs/lassen.yml

@@ -57,10 +57,10 @@ variables:
   parallel:
     matrix:
       - PYOMP_CI_PYTHON_VERSION:
-        - "3.8"
         - "3.9"
         - "3.10"
         - "3.11"
+        - "3.12"
 
 build-pyomp-lassen:
   extends: [.base-job, .python-variants]