fix: Multi output grouping by namespace

What this PR does / why we need it?

This PR fixes output overriding when using multi mode and grouping by namespace (default!). Assuming multiple unused resource types exist, only the last in the list will be displayed.

Regardless to the misfortunate output:

Grouping by resource works as expected for all resources
Deletion works as expected for all resources

Example

```console $ kor sa,cm -n kor --group-by=resource kor version: vdev _ _____ ____ | |/ / _ \| _ \ | ' / | | | |_) | | . \ |_| | _ < |_|\_\___/|_| \_\ Unused ServiceAccounts: +---+-----------+---------------+ | # | NAMESPACE | RESOURCE NAME | +---+-----------+---------------+ | 1 | kor | test | +---+-----------+---------------+ Unused ConfigMaps: +---+-----------+---------------+ | # | NAMESPACE | RESOURCE NAME | +---+-----------+---------------+ | 1 | kor | test | +---+-----------+---------------+ ```

Before fix

```console $ kor sa,cm -n kor --group-by=namespace kor version: vdev _ _____ ____ | |/ / _ \| _ \ | ' / | | | |_) | | . \ |_| | _ < |_|\_\___/|_| \_\ Unused resources in namespace: "kor" +---+---------------+---------------+ | # | RESOURCE TYPE | RESOURCE NAME | +---+---------------+---------------+ | 1 | ConfigMap | test | +---+---------------+---------------+ ```

After fix

```console $ kor sa,cm -n kor --group-by=namespace kor version: vdev _ _____ ____ | |/ / _ \| _ \ | ' / | | | |_) | | . \ |_| | _ < |_|\_\___/|_| \_\ Unused resources in namespace: "kor" +---+----------------+---------------+ | # | RESOURCE TYPE | RESOURCE NAME | +---+----------------+---------------+ | 1 | ServiceAccount | test | | 2 | ConfigMap | test | +---+----------------+---------------+ ```

PR Checklist

[ ] This PR adds K8s exceptions (false positives)
[x] This PR adds new code
[ ] This PR includes tests for new/existing code
[ ] This PR adds docs

GitHub Issue

Closes #348

Notes for your reviewers

The root cause relies in the diff loop, and is outsourced to the namespace loop in case that's the requested output mode.

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Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 42.42%. Comparing base (c70a3ac) to head (f1c8cd0). Report is 4 commits behind head on main.

Files	Patch %	Lines
pkg/kor/multi.go	0.00%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #349 +/- ## ========================================== - Coverage 42.45% 42.42% -0.03% ========================================== Files 61 61 Lines 3239 3241 +2 ========================================== Hits 1375 1375 - Misses 1644 1646 +2 Partials 220 220 ```

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yonahd / kor