Move to Lucene 9.12's new PostingsFormat.

[jpountz]

A while back, Lucene changed the way that it encodes doc IDs from PFOR-delta to FOR-delta, which is a bit faster but less space-efficient. In order to avoid introducing space-efficiency regressions (especially on dense postings lists, which are common on Logging datasets), @iverase moved Elasticsearch to a copy of the Lucene postings format that would still use PFOR-delta for compression. (#103601)

But Lucene 9.12 introduced a new postings format that has better skipping logic (in general). It would be nice to take advantage of it. I would suggest the following plan:

• Use Lucene912PostingsFormat on indexes whose storage efficiency is not critical (heuristic to be defined, e.g. when index.codec is default and source.mode is not synthetic?).
• Create a new postings format that is a copy of Lucene912PostingsFormat but with a more space-efficient encoding of doc deltas. @dnhatn and I played with it earlier this year, there is room for significant improvement by storing exceptions (the P from PFOR stands for "patched") more efficiently and allowing more exceptions per block.
• Move indexes whose storage efficiency is important to this new postings format instead of ES812PostingsFormat.
• Disallow using ES812PostingsFormat on new indexes.
• Move the write logic of ES812PostingsFormat to the test folder.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)