Fix feature state for Scheduling Framework

It was declared GA in 1.19
https://github.com/kubernetes/enhancements/blob/master/keps/sig-scheduling/624-scheduling-framework/kep.yaml

Author: alculquicondor

content/en/docs/concepts/scheduling-eviction/scheduling-framework.md

@@ -8,7 +8,7 @@ weight: 90
 
 <!-- overview -->
 
-{{< feature-state for_k8s_version="v1.15" state="alpha" >}}
+{{< feature-state for_k8s_version="v1.19" state="stable" >}}
 
 The scheduling framework is a pluggable architecture for the Kubernetes scheduler.
 It adds a new set of "plugin" APIs to the existing scheduler. Plugins are compiled into the scheduler. The APIs allow most scheduling features to be implemented as plugins, while keeping the