On many analyzers, the getDefaultStopSet() method returns a modifiable set, contrary to the docs

### Description

Several of the analyzers state that they are supposed to return an unmodifiable `CharArraySet`, but the set that is returned is writable, as you can see in the source.

https://github.com/apache/lucene/blob/cc342ea7407c729a743123d8f7957aff6c6f9792/lucene/analysis/common/src/java/org/apache/lucene/analysis/fr/FrenchAnalyzer.java#L67-L92

Note that the `Wordlistloader.getWordSet()` method also returns a writable `CharArraySet` and it is stored in static fields in several analyzers.

### Version and environment details

All versions, all environments

Start with the referenced FrenchAnalyzer.java implementation and inspect Wordlistloader.getWordSet(), then trace the other analyzers that store or return these sets. Confirm the documented behavior for getDefaultStopSet() and identify the affected analyzer implementations. Done means the returned CharArraySet instances are unmodifiable as documented across the affected analyzers.

Written by the indexing model from the issue text.