Add 'legacy' charm templates

Treat branches named 'legacy' as if they were jammy-based.

Change-Id: I10e32f1be492bfc868a2308ecb82e3537ae452b3
Signed-off-by: Brian Haley <haleyb.dev@gmail.com>

zuul.d/jobs.yaml

@@ -42,6 +42,7 @@
       - ^.*/jammy
       - ^.*/yoga
       - ^.*/2024.1
+      - legacy
 
 - job:
     name: openstack-tox-pep8-charm-noble

zuul.d/project-templates.yaml

@@ -83,6 +83,7 @@
               - stable/jammy
               - stable/yoga
               - stable/2024.1
+              - legacy
         - openstack-tox-cover-charm-noble:
             branches:
               - stable/noble
@@ -137,3 +138,19 @@
       jobs:
         - openstack-tox-pep8-charm-jammy
         - openstack-tox-py310
+
+- project-template:
+    name: openstack-python3-charm-jobs-legacy
+    description: |
+      Runs unit tests for an OpenStack Charm project under the CPython
+      version 3 releases designated for testing across OpenStack
+      releases on 'legacy' stable branches. These are the same jobs
+      as stable/2024.1.
+    check:
+      jobs:
+        - openstack-tox-pep8-charm-jammy
+        - openstack-tox-py310
+    gate:
+      jobs:
+        - openstack-tox-pep8-charm-jammy
+        - openstack-tox-py310