🌱 Makefile, samples: minor cleanup/fixes

[joelanford]

• remove the Extension CR sample (we recently removed that entire API and controller from the project)
• in the ClusterExtension CR sample, add .spec.installNamespace (we recently added that as a required field)
• In the Makefile, allow KIND_CLUSTER_VERSION to be provided from the environment

Description

Reviewer Checklist

• [ ] API Go Documentation
• [ ] Tests: Unit Tests (and E2E Tests, if appropriate)
• [ ] Comprehensive Commit Messages
• [ ] Links to related GitHub Issue(s)

[netlify[bot]]

✅ Deploy Preview for olmv1 ready!

Name	Link
🔨 Latest commit	a7e83e4b353d5bb4695273b0d2449d0364d0c5e1
🔍 Latest deploy log	https://app.netlify.com/sites/olmv1/deploys/66465f09d278d80008505d40
😎 Deploy Preview	https://deploy-preview-866--olmv1.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[openshift-ci[bot]]

@skattoju: changing LGTM is restricted to collaborators

In response to [this](https://github.com/operator-framework/operator-controller/pull/866#pullrequestreview-2061719926): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.18%. Comparing base (55c19a1) to head (a7e83e4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #866 +/- ## ======================================= Coverage 83.18% 83.18% ======================================= Files 15 15 Lines 874 874 ======================================= Hits 727 727 Misses 102 102 Partials 45 45 ``` | [Flag](https://app.codecov.io/gh/operator-framework/operator-controller/pull/866/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=operator-framework) | Coverage Δ | | |---|---|---| | [e2e](https://app.codecov.io/gh/operator-framework/operator-controller/pull/866/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=operator-framework) | `62.24% <ø> (ø)` | | | [unit](https://app.codecov.io/gh/operator-framework/operator-controller/pull/866/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=operator-framework) | `74.93% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=operator-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.