fix listing events for external clusters

[xrstf]

What this PR does / why we need it:
Since controller-runtime introduced the new caching config, we had to explicitly enable caches per namespace. Looking at our dev system, I spotted

{"level":"error","time":"2023-10-26T06:36:58.837Z","caller":"handler/routing.go:153","msg":"unable to list: default because of unknown namespace for the cache","request":"/api/v2/projects/wt8bxznl97/kubernetes/clusters/2g6zd8vwkw/events"}

Looking deeper, it seemed that we accidentally specified a namespace when ExternalClusters are not namespaced.

Fixes #6331

We can backport this, even though the error should only surface with controller-runtime 0.16, i.e. KKP 2.24.

What type of PR is this?
/kind bug

Does this PR introduce a user-facing change? Then add your Release Note here:

Fix listing events for external clusters

Documentation:

NONE

[pkprzekwas]

/approve
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 9084c6f388724b6062cdfcbe501dcace5dc910c9

[xrstf]

/cherrypick release/v2.23

[kubermatic-bot]

@xrstf: once the present PR merges, I will cherry-pick it on top of release/v2.23 in a new PR and assign it to you.

In response to this:

/cherrypick release/v2.23

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[xrstf]

/cherrypick release/v2.22

[kubermatic-bot]

@xrstf: once the present PR merges, I will cherry-pick it on top of release/v2.22 in a new PR and assign it to you.

In response to this:

/cherrypick release/v2.22

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[ahmedwaleedmalik]

/approve

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmedwaleedmalik, pkprzekwas

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• modules/api/pkg/handler/OWNERS [ahmedwaleedmalik,pkprzekwas]

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[kubermatic-triage-bot]

/retest
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[embik]

/retest

[embik]

/retest

[kubermatic-triage-bot]

/retest
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[kubermatic-bot]

@xrstf: new pull request created: #6342

In response to this:

/cherrypick release/v2.22

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[kubermatic-bot]

@xrstf: new pull request created: #6343

In response to this:

/cherrypick release/v2.23

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