chore: remove generate-release-notes command and supporting files (#6084)

Removes the generate-release-notes slash command and the files that existed
solely to support it:

- .ai/commands/generate-release-notes.md (the command/rule)
- scripts/fetch-jira-tickets.js (referenced only by the command)
- docs/release-notes/RELEASE_NOTES_TEMPLATE.md (referenced only by the command)

Author: dantovska