Refresh supported operating systems list

* Declare full support for distributions OpenStack supports in Yoga [1],
  with an exception of CentOS Stream 8.
* Deprecate support for distributions that only ship Python 3.6.
* Deprecate Fedora support. Being a moving target, it is hard to chase.
  Users that prefer a Red Hat system are better served by CentOS Stream.

[1] https://governance.openstack.org/tc/reference/runtimes/yoga.html

Change-Id: Iad2c27d72da738163f0e14c00d57a7962b82b82e

doc/source/install/index.rst

@@ -12,32 +12,23 @@ using Bifrost.
 Supported operating systems
 ===========================
 
-1st tier support (fully tested in the CI, no known or potential issues):
+Full support (fully tested in the CI, no known or potential issues):
 
 * CentOS Stream 8
+
+  .. note::
+     RHEL 8 and regular CentOS 8 are assumed to work but not tested explicitly.
+
 * Ubuntu 20.04 "Focal"
 * Debian 11 "Bullseye"
 
-2nd tier support (limited testing):
+Deprecated, support may be removed at any moment:
 
 * Ubuntu 18.04 "Bionic"
 * Debian 10 "Buster
-
-  Tested in the Bifrost CI only as non-voting jobs (does not block CI in case
-  of failure).
-
-* RHEL 8 and regular CentOS 8
-
-  Only tested indirectly via CentOS Stream 8.
-
 * openSUSE Leap 15.2
-
-  Tested in the CI but has frequent issues.
-
 * Fedora 34 (32+ is supported but not recommended)
 
-  Only the latest Fedora is tested in the CI.
-
 .. note::
    Operating systems evolve and so does the support for them, even on stable
    branches. This especially concerns Fedora, which is evolving faster than

releasenotes/notes/yoga-versions-4a460f5f9691b37f.yaml

@@ -0,0 +1,8 @@
+---
+deprecations:
+  - |
+    Support for distributions using Python 3.6, namely Ubuntu Bionic, Debian 10
+    "Buster" and openSUSE 15.2/15.3, has been deprecated and may be removed
+    at any moment.
+  - |
+    Support for Fedora has been deprecated, please use CentOS Stream 8.