Fix stale block size comments in Lucene104 postings reader/writer

[vukasin-djuricic]

Fixes #15670.

BLOCK_SIZE went from 128 to 256 in 10.4, but a few comments still spell out
values computed with the old block size:

• Lucene104PostingsWriter: level 1 skip data is written every 32 blocks, which
  is 8,192 docs, not 4,096.
• Lucene104PostingsWriter: BLOCK_SIZE * Integer.SIZE / Long.SIZE is 128, not 64.
  The assert on the next line already uses BLOCK_SIZE / 2.
• Lucene104PostingsReader: BLOCK_SIZE / Long.SIZE is 4, not 2.

This is a follow-up to #15672, where the question was raised whether other
comments needed the same fix. I went through the lucene104 package looking
for comments that hard-code values derived from BLOCK_SIZE, and these three
are the only ones left. The version history in package-info.java is correct
as is, since it describes the 9.12 format where BLOCK_SIZE was still 128.

Documentation only, no change in behaviour, so no CHANGES.txt entry.