CI: Adjust Grafana versions, bump to 9.0.0 for test environment

Author: amotl

CHANGES.rst

@@ -5,7 +5,7 @@ grafana-wtf changelog
 
 in progress
 ===========
-- CI: Use most recent Grafana 7.5.16, 8.5.5, and 9.0.0-beta3
+- CI: Use most recent Grafana 7.5.16, 8.5.6, and 9.0.0
 - Fix dashboard exploration when the ``annotations.list`` slot is ``None``
   instead of an empty list. Thanks, @TaylorMutch!
 

tests/.env

@@ -1,3 +1,4 @@
 #GRAFANA_VERSION=6.7.6
-#GRAFANA_VERSION=7.5.15
-GRAFANA_VERSION=8.4.4
+#GRAFANA_VERSION=7.5.16
+#GRAFANA_VERSION=8.5.6
+GRAFANA_VERSION=9.0.0