[CI] Update CertManager instances from 1.1.0 to 1.6.x

Summary

In #435 (closed), we updated CertManager to 1.6.1, which removed some deprecated APIs.

At the moment, CI clusters are running CertManager 1.1.0.

I deployed the changes in !330 (merged) to a personal cluster running CertManager 1.1.0, and everything worked as expected - from the source API definitions, it appears all of the v1 APIs were available in 1.1.0.

That said, 1.1.0 is a bit old now (November 2020), so we should get that updated as soon as possible.

https://cert-manager.io/docs/installation/upgrading

Acceptance criteria

• All CI clusters are running CertManager 1.6.x (can use scripts/install_certmanager.sh, updated in !330 (merged))