Remove kbn-version header from requests to Kibana. (#14482) (#14493)

Non-browser clients are expected to use only kbn-xsrf for CSRF protection, and not kbn-version.
kbn-version is problematic when Kibana is upgraded, since the version is only pinned at startup when libbeat establishes the connection.

See elastic/kibana@621bf0e

Fixes #14481

libbeat/kibana/client.go

@@ -194,9 +194,6 @@ func (conn *Connection) Send(method, extraPath string,
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Add("Accept", "application/json")
 	req.Header.Set("kbn-xsrf", "1")
-	if method != "GET" {
-		req.Header.Set("kbn-version", conn.Version.String())
-	}
 
 	for header, values := range headers {
 		for _, value := range values {