Deprecate Prometheus 1.x

Deprecates support for Prometheus v1.x.
In Xena support for it will be removed from Kolla Ansible.

Change-Id: I027b19621196c698e09f79af294ba1b5dbfc0516

ansible/roles/prometheus/defaults/main.yml

@@ -118,6 +118,7 @@ prometheus_blackbox_exporter_endpoints: []
 ####################
 
 # Whether to keep using Prometheus server v1 (due to no data-preserving migration path to v2)
+# NOTE(piotrp): Deprecated in Wallaby and will be removed in Xena
 prometheus_use_v1: no
 
 ####################

doc/source/reference/logging-and-monitoring/prometheus-guide.rst

@@ -36,6 +36,10 @@ However, if you want to stay on 1.x series, set the following variable:
 
    prometheus_use_v1: yes
 
+.. warning::
+   Support for Prometheus 1.x is deprecated and will be removed in next
+   Kolla Ansible release (Xena).
+
 In order to remove leftover volume containing Prometheus 1.x data, execute:
 
 .. code-block:: console

releasenotes/notes/deprecate-prometheus-v1-c174f03d62c46795.yaml

+---
+deprecations:
+  - |
+    Deprecates support for Prometheus v1.x. In Xena release cycle
+    support for this image will be removed from Kolla Ansible.