Drop centos-7 and ubuntu-xenial testing

This is in preparation for future cleanup of these distros as test
platforms for opendev.

Change-Id: I99bdc0c051b5d27b48c0bed28ec635cea598f66f

.zuul.yaml

@@ -10,21 +10,11 @@
       zuul_work_dir: '{{ zuul.project.src_dir }}'
       zuul_work_virtualenv: '{{ ansible_user_dir }}/.venv'
 
-- job:
-    name: bindep-centos-7
-    parent: bindep-base
-    nodeset: centos-7
-
 - job:
     name: bindep-centos-8-stream
     parent: bindep-base
     nodeset: centos-8-stream
 
-- job:
-    name: bindep-ubuntu-xenial
-    parent: bindep-base
-    nodeset: ubuntu-xenial
-
 - job:
     name: bindep-ubuntu-bionic
     parent: bindep-base
@@ -42,9 +32,7 @@
       - publish-opendev-nox-docs
     check:
       jobs:
-        - bindep-centos-7
         - bindep-centos-8-stream
-        - bindep-ubuntu-xenial
         - bindep-ubuntu-bionic
         - bindep-ubuntu-focal
         - build-python-release
@@ -58,9 +46,7 @@
         - nox-cover
     gate:
       jobs:
-        - bindep-centos-7
         - bindep-centos-8-stream
-        - bindep-ubuntu-xenial
         - bindep-ubuntu-bionic
         - bindep-ubuntu-focal
         - build-python-release