cert-manager: Switch to the new cmctl repository

[inteon]

The cert-manager command line tool has been moved to its own separate repository (https://github.com/cert-manager/cmctl).
This PR updates the krew index to point to this new repository.

[ahmetb]

🤖 Beep beep! I’m a robot speaking on behalf of @ahmetb. 🤖

---

This pull request does not seem to be a straightforward version bump. I'll have a human review this. If we don't respond within a day, feel free to ping us.

Why wasn't this detected as a plugin version bump:

file plugins/cert-manager.yaml is not a straightforward version bump: version should have metadata ("alpha.0"), only no pre-release versions are auto-approved
/kind plugin-update

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: inteon
Once this PR has been reviewed and has the lgtm label, please assign chriskim06 for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ahmetb]

We don't do prerelease versions.

[inteon]

We don't do prerelease versions.

I updated the PR to point to the v2.1.0 release now that it has been released.