operator: failed to upgrade from 19.2.12 to 20.1.15 on n1-standard-4

I had a 3 node `n1-standard-4` cluster with

```yaml
accessModes:
- ReadWriteOnce
resources:
requests:
storage: "60Gi"
volumeMode: Filesystem
resources:
requests:
cpu: "2"
memory: "4Gi"
limits:
cpu: "2"
memory: "4Gi"
```

settings, I cannot upgrade successfully, can't tell whether it's a resource issue or something wrong with `20.1.15`. I did verify it was specified in the manifest.