Replace deprecated perl-style regex

Zuul deprecated usage of perl-style regex.

```
  All regular expressions must conform to RE2 syntax, but an
  expression using the deprecated Perl-style syntax has been detected.
  Adjust the configuration to conform to RE2 syntax.
```

Change-Id: Id9ef016f799ae2bdf27b6c5022b6bec662bec74e

.zuul.yaml

@@ -2,7 +2,9 @@
     name: glance-secure-rbac-protection-functional
     parent: devstack-tempest
     timeout: 4200
-    branches: ^(?!stable/wallaby).*$
+    branches:
+      regex: ^stable/wallaby$
+      negate: true
     required-projects:
       - openstack/devstack-gate
       - openstack/glance