G1: Incorrect reference discovery MT degree in concurrent marking

In `src/hotspot/share/gc/g1/g1CollectedHeap.cpp`:

```
_ref_processor_cm =
    new ReferenceProcessor(&_is_subject_to_discovery_cm,
                           ParallelGCThreads, // degree of mt processing
                           (ParallelGCThreads > 1) || (ConcGCThreads > 1), // mt discovery
                           MAX2(ParallelGCThreads, ConcGCThreads), // degree of mt discovery
                           false, // Reference discovery is not atomic
                           &_is_alive_closure_cm); // is alive closure
```

CM ref processor does ref discovery during concurrent marking, which use `ConcGCThreads` GC threads. Therefore, the MT degree is determined by `ConcGCThreads`; `ParallelGCThreads` should not be used.