fix(datasource): add warning when unable to find datasource (#20805)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

lib/modules/datasource/index.ts

@@ -278,6 +278,7 @@ async function fetchReleases(
   const datasource = getDatasourceFor(datasourceName);
   // istanbul ignore if: needs test
   if (!datasource) {
+    logger.warn({ datasource: datasourceName }, 'Unknown datasource');
     return null;
   }
   registryUrls = resolveRegistryUrls(