Security: Use Header.Set and Header.Del for X-Grafana-User header (#25495)

This ensures that the X-Grafana-User header can be trusted.
If the configuration enabled the setting of this header, the
server can now trust that X-Grafana-User is set/unset by Grafana.
Before this, an anonymous user could simply set the X-Grafana-User
header themselves (using the developer tool for example)

(cherry picked from commit 034abaa73a)

pkg/api/pluginproxy/ds_proxy.go

@@ -187,9 +187,7 @@ func (proxy *DataSourceProxy) getDirector() func(req *http.Request) {
 			req.Header.Add("Authorization", dsAuth)
 		}
 
-		if proxy.cfg.SendUserHeader && !proxy.ctx.SignedInUser.IsAnonymous {
-			req.Header.Add("X-Grafana-User", proxy.ctx.SignedInUser.Login)
-		}
+		applyUserHeader(proxy.cfg.SendUserHeader, req, proxy.ctx.SignedInUser)
 
 		keepCookieNames := []string{}
 		if proxy.ds.JsonData != nil {