Remove 3.13 from ltsVersions (#3016)

3.13 is not released yet; listing it as an LTS version breaks CI in
prometheus/prometheus.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Author: krajorama

docs-config.ts

@@ -46,7 +46,7 @@ export default {
 
   // Long-term support versions configuration.
   ltsVersions: {
-    prometheus: ["3.5", "3.13"],
+    prometheus: ["3.5"],
   },
 
   // Repositories for the downloads page. The order in this file is the