Replace “master” icon with control plane

[sftim]

https://github.com/kubernetes/community/tree/master/icons#kubernetes-icons-set has an icon representing the kubernetes control plane, labelled “master”

I recommend replacing this with a group symbol (possibly not a heptagon) to represent the set of control plane elements.

The term “master” is problematic because the control plane doesn't have to run on a single primary host with replicas, and also because in some cultures it implies references to slavery.

Relevant to kubernetes/website#20767

/wg naming

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[nikhita]

/remove-lifecycle stale

[justaugustus]

Recommendation in-flight here: #5243

[justaugustus]

/assign @celestehorgan
/sig docs