Rename mergeThreadCount in ConcurrentMergeScheduler to avoid clashing with method of the same name

[cwperks]

Description

Opening up this small PR to rename mergeThreadCount (the field) to mergeThreadCounter to avoid clashing with a method in the same class also named mergeThreadCount

This variable is used to assign merge thread numbers by incrementing from 0 so I thought mergeThreadCounter may be a more appropriate name.

Resolves: #13114

Below is a snippet of the current state of the class where the field and method have the same name.

/** How many {@link MergeThread}s have kicked off (this is use to name them). */
protected int mergeThreadCount;

/**
 * Returns the number of merge threads that are alive, ignoring the calling thread if it is a
 * merge thread. Note that this number is ≤ {@link #mergeThreads} size.
 *
 * @lucene.internal
 */
public synchronized int mergeThreadCount() {
  Thread currentThread = Thread.currentThread();
  int count = 0;
  for (MergeThread mergeThread : mergeThreads) {
    if (currentThread != mergeThread
        && mergeThread.isAlive()
        && mergeThread.merge.isAborted() == false) {
      count++;
    }
  }
  return count;
}