Upgrades Nextcloud from 34 to 35 (major version upgrade).

IMPORTANT:
- Do not skip major versions: your instance must be on the latest 34.x before
  upgrading to 35.
- Use `check_major_upgrade` (`abra app cmd <app-name> app check_major_upgrade`)
  to check whether an instance is ready to upgrade to the next Nextcloud
  major version.
- Nextcloud does NOT support downgrades. Take a backup before deploying.
- After deploying, check the logs and run any pending repair/upgrade steps:
  `abra app cmd <app> app run_occ '"app:update --all"'`
- Review app (plug-in) compatibility with Nextcloud 35 before upgrading; some
  apps may need to be updated or temporarily disabled.