(build) strange "/generated" folders show up out of nowhere [LUCENE-9520]

Precommit has been failing for me recently with odd exceptions like this:

```Java
* What went wrong:
Execution failed for task ':checkWorkingCopyClean'.
> Working copy is not a clean git checkout (skip with -Pvalidation.git.failOnModified=false), offending files:
- lucene/analysis/common/src/java/generated (untracked)
- lucene/codecs/src/java/generated (untracked)
- lucene/core/src/java/generated (untracked)
- lucene/highlighter/src/java/generated (untracked)
- lucene/highlighter/src/test/generated_tests (untracked)
- lucene/memory/src/java/generated (untracked)
- lucene/queries/src/java/generated (untracked)
- lucene/queryparser/src/java/generated (untracked)
- lucene/sandbox/src/java/generated (untracked)
- lucene/test-framework/src/java/generated (untracked)
```

Don't know where the "generated" folder comes from. IntellIJ is the primary suspect but I'm not 100% sure.

---
Migrated from [LUCENE-9520](https://issues.apache.org/jira/browse/LUCENE-9520) by Dawid Weiss (@dweiss)

Start by reproducing the precommit failure and inspecting the listed untracked generated directories. Trace which build or development process creates them, with IntelliJ mentioned as a possible source. Done means the unexpected directories no longer appear and checkWorkingCopyClean passes without disabling validation.

Written by the indexing model from the issue text.