Make LINK splitMethod the default for SplitShardCmd

REWRITE splitMethod is still the default in SplitShardCmd [1], despite LINK being much faster. IndexSizeTrigger in branch_8x already uses LINK by default [2], and we have found LINK to be reliable and performant at scale. This work will just update the default in SplitShardCmd to make LINK the default overall.

 [1]https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/cloud/api/collections/SplitShardCmd.java#L88

 [2]https://github.com/apache/lucene-solr/blob/branch_8x/solr/core/src/java/org/apache/solr/cloud/autoscaling/IndexSizeTrigger.java#L186