Use bools for ignorable in EndTxnResponse.json

[tomduckering]

A tiny fix to a single protocol JSON which was recently updated under KAFKA-14562. It seems to erroneously use strings where bools appear to be the right type.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)

[tomduckering]

A JIRA bug didn't seem appropriate for something so trivial but happy to raise one if that's the right thing to do.

[tomduckering]

@jolshan - FYI since this minor issue came in with PR16719