[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

Change-Id: I37da1259d85c03b66c87e404511a2bdf69563acf

Author: karelyatin

zuul.d/job-templates.yaml

@@ -81,7 +81,7 @@
 
 - project-template:
     name: neutron-periodic-jobs
-    periodic:
+    periodic-weekly:
       jobs:
         - neutron-functional
         - neutron-functional-with-uwsgi-fips