Fix Zuul warnings with regexps

zuul now reports warnings when using perl-style regexp:
| The RE2 syntax error is: invalid perl operator: (?!
for
| branches: ^(?!stable)

This commit fixes it by using the correct RE2 syntax

more details: https://zuul-ci.org/docs/zuul/latest/project-config.html#regular-expressions

Change-Id: Iab9d4849d50c01ff35ce159a988fde95593f7ebf

zuul.d/projects.yaml

@@ -40,8 +40,10 @@
     periodic:
       jobs:
         - publish-trove-guest-image-ubuntu-jammy:
-            branches: ^(?!stable/.*).*$
-        - publish-trove-guest-image-ubuntu-jammy-dev:
-            branches: ^(?!stable/.*).*$
+            branches:
+                regex: ^stable/.*$
+                negate: true
         - publish-trove-datastore-images:
-            branches: ^(?!stable/.*).*$
+            branches:
+                regex: ^stable/.*$
+                negate: true