api4/license: Return 400 instead of 501 if format is not passed (#19856)

```release-note
NONE
```


Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/license.go

@@ -31,7 +31,7 @@ func getClientLicense(c *Context, w http.ResponseWriter, r *http.Request) {
 	format := r.URL.Query().Get("format")
 
 	if format == "" {
-		c.Err = model.NewAppError("getClientLicense", "api.license.client.old_format.app_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("getClientLicense", "api.license.client.old_format.app_error", nil, "", http.StatusBadRequest)
 		return
 	}