SOLR-18450: Use /api/collections in test_auth.bats

[serhiy-bzhezytskyy]

https://issues.apache.org/jira/browse/SOLR-18450

Description

test_auth.bats still calls GET /api/cluster, which SOLR-18450 moved to /api/collections, so its "enable auth connects via zookeeper" test gets a 404 on main.

Solution

The two calls in that test use /api/collections. Test only. Written with Claude Code.

Tests

./gradlew integrationTests --tests=test_auth.bats: 3 of 4 tests pass on main, 4 of 4 with this change.

Checklist

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change

[epugh]

Thanks guys, I missed this one!