Use RE2 compatible regex for branches

Zuul has deprecated the usage of negative regex[1],
this patch switches the branches regex to the supported
one.

[1] https://zuul-ci.org/docs/zuul/latest/project-config.html#regex

Closes-Bug: #2037034
Change-Id: I66dcb56ab5ba9cfdefbeb25f1d01871ea0d0d362
1 file changed