[unmaintained-only] Periodic jobs will be weekly only

As agreed in the Neutron CI meeting, the unmaintained branches periodic
jobs will be weekly only [1].

[1]https://meetings.opendev.org/meetings/neutron_ci/2024/neutron_ci.2024-09-23-14.01.log.html#l-97

Depends-On: https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/930594

Change-Id: I0bc95e3705bd3114c36e1ec95701da2e2d13b13f

Author: ralonsoh

zuul.d/job-templates.yaml

@@ -99,7 +99,7 @@
 
 - project-template:
     name: neutron-periodic-jobs
-    periodic:
+    periodic-weekly:
       jobs:
         - neutron-functional
         - neutron-fullstack