G1: Rework VM_G1Concurrent VMOp to clearly identify it as pause

Our support engineers asked this:

> I see these G1Concurrent safepoints in JDK17:
> [0.064s][info][safepoint ] Safepoint "G1Concurrent", Time since last: 1666947 ns, Reaching
safepoint: 79150 ns, At safepoint: 349999 ns, Total: 429149 ns
> I've always thought that "concurrent" and "safepoint" are basically antonyms.
> What is a G1Concurrent safepoint? How can a concurrent event require a safepoint?

I think the naming here is unfortunate:
https://github.com/openjdk/jdk/blob/b79554bb5cef14590d427543a40efbcc60c66548/src/hotspot/share/gc/g1/g1VMOperations.hpp#L89-L103

...and we can clean that up a bit. I'll propose a patch.