Remove ansible version pin at 2.8

In the past, we've had to pin ansible to a specific version to
avoid certain behaviors, since we have since updated to a newer
version past 2.8 now for quite some time, there's no need to
continue pinning it.

Change-Id: Icf8a3672c630ee9f93c0c727182832467ec7af42

zuul.d/base.yaml

@@ -108,11 +108,6 @@
     dependencies:
       - name: opendev-buildset-registry
     abstract: true
-    # Minimum version required is 2.8.
-    # When default in zuul moves to 2.8, remove
-    # this line.
-    # See also: https://zuul-ci.org/docs/zuul/user/config.html#attr-job.ansible-version
-    ansible-version: 2.8
     description: |
       This job is building a docker image for
       OpenStack-Helm usage.
@@ -136,7 +131,6 @@
     dependencies:
       - name: opendev-buildset-registry
     abstract: true
-    ansible-version: 2.8
     secrets:
       name: docker_credentials
       secret: openstack-helm-dockerhub
@@ -146,7 +140,6 @@
     name: openstack-helm-images-promote
     parent: promote-docker-image
     abstract: true
-    ansible-version: 2.8
     secrets:
       name: docker_credentials
       secret: openstack-helm-dockerhub