Add support for k8s 1.18 and 1.19

At the moment the operator cannot be installed in k8s 1.18.

The operator's init job fails with the error

+ kubectl delete csr --ignore-not-found stackgres-operator
certificatesigningrequest.certificates.k8s.io "stackgres-operator" deleted
+ cat
+ openssl req -new -nodes -text -keyout /tmp/root.key -subj /CN=stackgres-operator.stackgres.svc -out /tmp/server.csr -config /tmp/csr.conf
Generating a RSA private key
....................+++++
.....................................................................+++++
writing new private key to '/tmp/root.key'
-----
+ cat
+ kubectl create -f -
++ cat /tmp/server.csr
++ base64
++ tr -d '\n'
certificatesigningrequest.certificates.k8s.io/stackgres-operator created
+ kubectl get csr stackgres-operator -o yaml
+ grep -q '^    type: Approved$'
+ kubectl certificate approve stackgres-operator
No resources found
Error from server (Forbidden): certificatesigningrequests.certificates.k8s.io "stackgres-operator" is forbidden: user not permitted to approve requests with signerName "kubernetes.io/legacy-unknown"