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GH-34175: [Docs] Remove Jira from .github/CONTRIBUTING.md (#34205)

### Rationale for this change

We no longer use Jira.

### What changes are included in this PR?

Recommend GitHub issues for questions, bugs, and feature requests, instead of Jira. GitHub prefers to show this file over others when present.
* Closes: #34175

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>