### Description

Today, [force merge is assigned one thread](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-threadpool.html) only.  

> For [force merge](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-forcemerge.html) operations. Thread pool type is fixed with a size of 1 and an unbounded queue size.

In a large cluster where there are many processors available, there could be a backlog of force merge queued up.  Perhaps we should look into changing the default of force merge thread_pool to a max of `min(1, (# of allocated processors) / 8)`
server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
public String getType() {
            return type;
        }
        ThreadPoolType(String type) {
            this.type = type;
        }
        private static final Map<String, ThreadPoolType> TYPE_MAP = Arrays.stream(ThreadPoolType.values())
            .collect(Collectors.toUnmodifiableMap(ThreadPoolType::getType, Function.identity()));
        public static ThreadPoolType fromType(String type) {
            ThreadPoolType threadPoolType = TYPE_MAP.get(type);
            if (threadPoolType == null) {
                throw new IllegalArgumentException("no ThreadPoolType for " + type);
            }
            return threadPoolType;
        }
    }
    public static final Map<String, ThreadPoolType> THREAD_POOL_TYPES = Map.ofEntries(
        entry(Names.SAME, ThreadPoolType.DIRECT),
        entry(Names.GENERIC, ThreadPoolType.SCALING),
        entry(Names.GET, ThreadPoolType.FIXED),
        entry(Names.ANALYZE, ThreadPoolType.FIXED),
        entry(Names.WRITE, ThreadPoolType.FIXED),
        entry(Names.SEARCH, ThreadPoolType.FIXED),
        entry(Names.SEARCH_COORDINATION, ThreadPoolType.FIXED),
        entry(Names.MANAGEMENT, ThreadPoolType.SCALING),
        entry(Names.FLUSH, ThreadPoolType.SCALING),
        entry(Names.REFRESH, ThreadPoolType.SCALING),
        entry(Names.WARMER, ThreadPoolType.SCALING),
        entry(Names.SNAPSHOT, ThreadPoolType.SCALING),
        entry(Names.SNAPSHOT_META, ThreadPoolType.SCALING),