Migrate jira.xwiki.org to OpenProject

Move all XWiki issue tracking from jira.xwiki.org to OpenProject (op.xwiki.org), then switch Jira off.

• Plan: https://design.xwiki.org/xwiki/bin/view/Proposal/JiraToOpenProjectMigration
• Proposal: https://forum.xwiki.org/t/migration-plan-from-jira-xwiki-org-to-openproject/18971
• Handling of security issues (separate proposal and vote): https://forum.xwiki.org/t/report-security-issues-through-github-security-advisories-instead-of-jira/18951

Order: contrib projects first, in batches, once the OpenProject Jira migrator can import them; then XCOMMONS, XRENDERING and XWIKI together in one weekend, once the features they need are available; Jira read-only and internal after that, and switched off 3 months later.

One sub-task per workstream. Sub-tasks link to the work tracked elsewhere (extension issues, OpenProject feature requests). Milestones are announced in the forum proposal thread.