Drop builds for f31 and beyond due to Python 2 retirement.

Add stream deprecation notice to description.

Signed-off-by: Merlin Mathesius <mmathesi@redhat.com>

avocado.yaml

@@ -6,14 +6,15 @@ data:
   description: >-
     Avocado is a set of tools and libraries (what people call
     these days a framework) to perform automated testing.
+    NOTICE: This stream is deprecated--please use stream '52lts' or '69lts'.
   license:
     module:
     - MIT
   dependencies:
   - buildrequires:
-      platform: []
+      platform: [f28, f29, f30]
     requires:
-      platform: []
+      platform: [f28, f29, f30]
   references:
     community: http://avocado-framework.github.io/
     documentation: http://avocado-framework.readthedocs.io/