Replace 'JIRA' with 'Jira' for EE-specific code comments

What does this MR do?

Replaces 'JIRA' with 'Jira' following the official representation.

cf. CE: gitlab-org/gitlab-ce!29849

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [n/a] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [n/a] Merge request performance guidelines
• Style guides
• [n/a] Database guides
• Separation of EE specific content

Closes #12498 (closed)