SOLR-17959: Remove issues key from changelog YAML

[andywebb1975]

https://issues.apache.org/jira/browse/SOLR-17959

Description

This removes the issues key from the changelog YAML, ref Jan's comment in SOLR-17959

Checklist

Please review the following and check all that apply:

• 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