Content-Type fixes in client4.ts and remote_cluster.go (#27887)

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Miguel de la Cruz <miguel@mcrx.me>

webapp/platform/client/src/client4.ts

@@ -4179,7 +4179,11 @@ export default class Client4 {
 
         let data;
         try {
-            data = await response.json();
+            if (headers.get('Content-Type') === 'application/json') {
+                data = await response.json();
+            } else {
+                data = await response.text();
+            }
         } catch (err) {
             throw new ClientError(this.getUrl(), {
                 message: 'Received invalid response from the server.',