[documentation] metrics-bind-addr should be removed from cluster-api-operator documentation

[jokuniew]

metrics-bind-addr was removed in v1.9.0 release

kubernetes-sigs/cluster-api#11140
https://github.com/kubernetes-sigs/cluster-api/blob/155e6604aea01770c979631265c057e5e7eff45e/CHANGELOG/v1.9.0.md

the flag's code was removed in
https://github.com/kubernetes-sigs/cluster-api-operator/pull/665/files

but cluster-api-operator documentation still reffers to it
https://github.com/kubernetes-sigs/cluster-api-operator/blob/main/docs/README.md?plain=1#L158

Just removing this one line from the documentation might not be enough; I believe that the documentation on collecting metrics should be revisited.

[k8s-ci-robot]

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