[centos6] update container for EOL

Change:
- Reference:
  https://lists.centos.org/pipermail/centos-devel/2020-December/056208.html
- Bump centos6 container to 1.25.0

Test Plan:
- ci_complete

Signed-off-by: Rick Elrod <rick@elrod.me>

changelogs/fragments/ansible-test-centos6-eol.yml

@@ -0,0 +1,2 @@
+minor_changes:
+  - ansible-test - centos6 end of life - container image updated to point to vault base repository (https://github.com/ansible/distro-test-containers/pull/54)

test/runner/completion/docker.txt

@@ -1,5 +1,5 @@
 default name=quay.io/ansible/default-test-container:1.9.1 python=3.6,2.6,2.7,3.5,3.7,3.8 seccomp=unconfined
-centos6 name=quay.io/ansible/centos6-test-container:1.8.0 python=2.6 seccomp=unconfined
+centos6 name=quay.io/ansible/centos6-test-container:1.26.0 python=2.6 seccomp=unconfined
 centos7 name=quay.io/ansible/centos7-test-container:1.8.0 python=2.7 seccomp=unconfined
 fedora29 name=quay.io/ansible/fedora29-test-container:1.9.4 python=3.7
 fedora30 name=quay.io/ansible/fedora30-test-container:1.9.2 python=3.7