Update metal3 chart cert-manager version

[hardys]

The current cert-manager version in the README is v1.11.1 which is now EOL, this version is also used in the GH workflow in CI, which could cause problems since according to the cert-manager docs it's compatible with 1.21 → 1.27 but in the CI workflow we're installing latest-stable k3s, e.g 1.29 at this point.

I would suggest we remove the version and install the latest in both the README and the CI workflow.