Fix zuul config warning

This change resolves the following warning detected by zuul.

  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.

  The RE2 syntax error is: invalid perl operator: (?!

Change-Id: I0c1be68030470b88dd4268d509e4c445667dc645

.zuul.yaml

@@ -51,7 +51,9 @@
       Run unit tests with main branch of SQLAlchemy, alembic and oslo.db.
       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/sqlalchemy/sqlalchemy
         override-checkout: main