refactor: use helper from controller runtime

[flavio]

Use a more concise way to handle "not found" errors that is made possible by using a controller runtime helper.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 71.78%. Comparing base (9c3dd61) to head (783f2bd).

Files	Patch %	Lines
internal/pkg/admission/policy-server-ca-secret.go	0.00%	3 Missing :warning:
...g/admission/policy-server-pod-disruption-budget.go	0.00%	3 Missing :warning:
controllers/admissionpolicy_controller.go	0.00%	1 Missing :warning:
internal/pkg/admission/policy-server-configmap.go	0.00%	1 Missing :warning:
internal/pkg/admission/policy-server-deployment.go	0.00%	1 Missing :warning:
internal/pkg/admission/policy-server-service.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #725 +/- ## ========================================== - Coverage 72.84% 71.78% -1.07% ========================================== Files 28 28 Lines 1812 1804 -8 ========================================== - Hits 1320 1295 -25 - Misses 373 388 +15 - Partials 119 121 +2 ``` | [Flag](https://app.codecov.io/gh/kubewarden/kubewarden-controller/pull/725/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewarden) | Coverage Δ | | |---|---|---| | [integration-tests](https://app.codecov.io/gh/kubewarden/kubewarden-controller/pull/725/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewarden) | `59.42% <45.00%> (-1.29%)` | :arrow_down: | | [unit-tests](https://app.codecov.io/gh/kubewarden/kubewarden-controller/pull/725/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=kubewarden) | `50.11% <20.00%> (-0.08%)` | :arrow_down: | 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=kubewarden#carryforward-flags-in-the-pull-request-comment) to find out more.

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[flavio]

@jvanz, @viccuad : I've updated golanglint-ci and then fixed all the warnings via 8d936333779b11f9cb2f1314341325b0f3af80a3. Please take a look at the commit description to see the reasons of the changes I've made

[flavio]

Note: this cannot be merged until the 1.12 release is done

[jvanz]

It seems there is something wrong, because tests and linter are failing. =(

[flavio]

@jvanz I'm a bit stuck with other tasks, would you be so kind to take this over once your PR about the owner reference is merged? :pray:

[jvanz]

@kubewarden/kubewarden-developers , I've updated this PR rebasing it on top of main and did some fixes to make the CI happy. To make all our CI green I had to update the controller-gen binary used. Otherwise, we get a nil pointer deref. That's why you can see so many line changes, I've commited the result of the generator after updating it. Let me know if you're fine with that. Otherwise, I can update the generator in another PR or commit the updated manifest files in another PR.

[jvanz]

@kubewarden/kubewarden-developers I've updated this branch rebasing it on top of main and squashing some commits to reduce the number of them.