Upgrade GatherPress to 0.36.0 once released

[bor0]

Follow-up to #1843. Once GatherPress 0.35.0 is out and we've upgraded to it, we should track the next release too.

0.36.0's GitHub milestone currently has no due date and 45 open issues (as of 2026-08-03), so it's not close to shipping yet — this issue is a placeholder to revisit once it does, rather than a scheduled upgrade for a specific date.

Plan

• Watch the 0.36.0 milestone / releases page for a stable 0.36.0 tag.
• Once released, review its changelog for anything affecting mu-plugins/wporg-groups-frontend, mu-plugins/groups/gatherpress-groups-tweaks.php, or the groups-site theme (settings/filter renames, block/API changes, minimum PHP/WP bumps).
• Deploy to a staging/testing group site first.
• Run PHPUnit (docker compose -f docker-compose.phpunit.yml up), covering the WordCamp Groups suite.
• Run E2E (npx playwright test / e2e-tests.yml workflow).
• Run the full manual groups-gatherpress-compat-test checklist (.claude/skills/groups-gatherpress-compat-test/SKILL.md): per-role browser pass, direct REST pass, GatherPress-off guard pass, security/perf sanity checks.
• Roll out network-wide once all of the above pass clean.

Acceptance criteria

• GatherPress is upgraded to the then-current 0.36.x network-wide.
• Automated (PHPUnit + E2E) and manual compat-checklist passes are all green, with results noted on this issue.
• No regressions in event creation/editing, RSVP, venue creation/map rendering, or member management across Organiser/Event Organiser/Member roles.