Warn before applying issue template
What does this MR do?
Adds a content override warning when applying another template.
Does this MR meet the acceptance criteria?
- Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
- Documentation created/updated or follow-up review issue created
- Code review guidelines
- Merge request performance guidelines
- Style guides
- Database guides
- Separation of EE specific content
Closes #16188 (closed)