fix: use node-fetch instead of native fetch

Signed-off-by: Oliver Dirks <oliver.dirks@gothaer.de>

.changeset/six-beds-exercise.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-defaults': patch
+---
+
+Use `node-fetch` instead of native fetch, as per https://backstage.io/docs/architecture-decisions/adrs-adr013

packages/backend-defaults/src/entrypoints/auth/plugin/PluginTokenHandler.ts

@@ -22,6 +22,7 @@ import { tokenTypes } from '@backstage/plugin-auth-node';
 import { JwksClient } from '../JwksClient';
 import { HumanDuration, durationToMilliseconds } from '@backstage/types';
 import { PluginKeySource } from './keys/types';
+import fetch from 'node-fetch';
 
 const SECONDS_IN_MS = 1000;