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: Id6b8fbf145b7e933ce2e82cebbe3d6c9531943b6

.zuul.yaml

@@ -5,7 +5,9 @@
       Build documentation for openstackdocstheme with main branch of important
       libs. Takes advantage of the base tox job's install-siblings feature.
     # The job only tests the latest and shouldn't be run on the stable branches
-    branches: ^(?!stable)
+    branches:
+      regex: ^stable/.*$
+      negate: true
     required-projects:
       - name: github.com/sphinx-doc/sphinx
     vars: