Update TieredMergePolicy's default floor segment size

[jpountz]

Lucene 10.2 increased the floor segment size from 2MB to 16MB via apache/lucene#14189. I'll copy the PR description here to explain the motivation:

My motivation is that such small segment sizes don't make index structures actually helpful vs. linear scans, so we should avoid them. Furthermore, there has been progress on merging rules for segments below the floor size, in particular merge policies no longer perform quadratic merging (apache/lucene#900) so this change will not make indexing/merging absurdly slow if an application flushes tiny segments.

Finally this likely helps vector search, which likes fewer segments better.

I believe that it would make sense for Elasticsearch to update MergePolicyConfig to align its defaults with Lucene?

[jpountz]

Another piece of information that may be interesting is that since this increases the floor segment size by about 10x, and the number of segments per tier is 10, this should result in about 10 fewer segments per shard.

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)