[v1.18] fix: verify argocd instance matches namespace label on appset cleanup

[jgwest]

What type of PR is this?
/kind bug

What does this PR do / why we need it:
Backport of argoproj-labs/argocd-operator#1971

[svghadi]

/lgtm
/approve

[svghadi]

/lgtm
/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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[jgwest]

/retest

cluster init failure

[openshift-ci]

@jgwest: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/v4.19-e2e	467022c	link	true	/test v4.19-e2e

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[svghadi]

v4.19-e2e failed again with a cluster init error. This test suite only runs basic validation on the default instance and the GitOpsService CR, and it’s passing on the other cherry-pick PRs. Ignoring the CI check and merging the PR.