Seal the BitSet class

### Description

`BitSet` is used in some performance-critical places. We should seal this class so that it only allows `FixedBitSet` and `SparseFixedBitSet` as sub-classes, to prevent applications from mistakenly hurting performance by creating other sub-classes that could make call sites of `BitSet#get`, `BitSet#nextSetBit` and others polymorphic?

Locate BitSet, FixedBitSet, and SparseFixedBitSet in the Java source and inspect their declarations and inheritance relationships first. Make BitSet permit only the two named subclasses, then run the relevant Lucene tests or build checks to confirm the change is valid.

Written by the indexing model from the issue text.