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

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

https://github.com/kubernetes-sigs/cluster-api/pull/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.

Start in docs/README.md around line 158 and review the linked v1.9.0 changelog and removal references for context. Check the surrounding metrics-collection documentation for other references to metrics-bind-addr, then update the documentation so it no longer describes the removed flag and accurately reflects the current guidance.

Written by the indexing model from the issue text.