Add a warning to the deprecated non-eventsource sync endpoint

Signed-off-by: Dominik Henneke <dominik.henneke@sda-se.com>

plugins/techdocs-backend/src/service/router.ts

@@ -127,6 +127,9 @@ export async function createRouter({
     await docsSynchronizer.doSync(
       () => {
         if (req.header('accept') !== 'text/event-stream') {
+          console.warn(
+            "The /sync/:namespace/:kind/:name endpoint but the call wasn't done by an EventSource. This behavior is deprecated and will be removed soon. Make sure to update the @backstage/plugin-techdocs package in the frontend to the latest version.",
+          );
           return createHttpResponse(res);
         }