MIGRATION: Define post-migration Jira updates

[toddfarmer]

If we migrate Jira content to GitHub issues, it probably makes sense to add a comment to the source Jira issue with a pointer to the migrated GitHub issue. This might also reference a document instructing users how to engage further after issue migration (e.g., manage their watched issues, assigned issues, stop working in Jira, etc.).

[toddfarmer]

It would be useful to identify a specific field in Jira we can leverage as a pointer to a migrated issue in GitHub. There is the "External Issue URL" field, which is sparsely used today, and mostly seems to have been used at one time to point to related GitHub PRs that are otherwise tracked via other means (comments, etc.). If we prefer a different field, we will need to either select an existing unused field, or request Infra create such a field and possibly add it to our issue screens.

[westonpace]

+1 for External Issue URL, I'm pretty sure it's not being regularly used today and I don't think we want to make this more complex than needed. I don't think the message needs to be too involved.

Arrow issues are now tracked on Github.  This issue has been migrated to https://github.com/apache/arrow/issues/xyz

[rok]

Comments pointing to migrated issues were added to all Jira issues, e.g. ARROW-17832.