Update jobs based on testing runtime for 2024.2

Based on [0] & [1] only py39, py311 and py312 (non-voting)
jobs are in the testing runtime for 2024.2, updated relevant
files accordingly.

[0] https://review.opendev.org/c/openstack/governance/+/908862
[1] https://governance.openstack.org/tc/reference/runtimes/2024.2.html

Change-Id: I8336691a59ef76f4ec2162d83d6b925c05497fad

.zuul.yaml

@@ -8,9 +8,6 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        - openstack-tox-py38:
-            required-projects:
-              - openstack/neutron
         - openstack-tox-py39:
             required-projects:
               - openstack/neutron
@@ -20,6 +17,9 @@
         - openstack-tox-py311:
             required-projects:
               - openstack/neutron
+        - openstack-tox-py312:
+            required-projects:
+              - openstack/neutron
         - openstack-tox-docs:
             required-projects:
               - openstack/neutron
@@ -28,9 +28,6 @@
         - neutron-tempest-plugin-vpnaas-ovn
     gate:
       jobs:
-        - openstack-tox-py38:
-            required-projects:
-              - openstack/neutron
         - openstack-tox-py39:
             required-projects:
               - openstack/neutron
@@ -55,6 +52,9 @@
         - neutron-vpnaas-openstack-tox-py310-with-sqlalchemy-main
     periodic-weekly:
       jobs:
+        - openstack-tox-py312:
+            required-projects:
+              - openstack/neutron
         - openstack-tox-py311:
             required-projects:
               - openstack/neutron