cgls/cgtop: spell field/column "CGroup" rather than "Control Group"

In the documentation we usually spell the concept "control group".
Internally in code we usually call it "cgroup" or "CGroup". In systemctl output we
called the field "CGroup" so far, i.e. a capitalized version of the
internal name. This is of course very unsystematic. Let's clean this up
a bit: let's now say:

* in docs, continue to spell it out "control groups"
* in brief output call it "CGroup"
* internally call it "cgroup" or "CGroup"

Fixes: #14429

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