Adjust required permissions for upgrade-dependencies.yaml workflow (gardener#309)

The `pull-request: write` permission is (and was) actually not required for this workflow. Instead, `id-token: write` will be required in the future because of a (yet) missing OIDC usage within the workflow.

Author: 8R0WNI3

.github/workflows/upgrade-dependencies.yaml

@@ -10,4 +10,4 @@ jobs:
     secrets: inherit
     permissions:
       contents: write
-      pull-requests: write
+      id-token: write