CLOS-6911: ELevate - add documentation coverage for CL8->CL9 upgrades on Plesk - #345

Plesk systems are now supported for CloudLinux 8 to 9 upgrades via the
Plesk cloudlinux8to9 tool.

- List Plesk under supported CL8->CL9 configurations and name both Plesk
  tools (cloudlinux7to8, cloudlinux8to9) in the FAQ.
- Rework the Plesk scenario to cover both tools, and download the latest
  tool release instead of pinning a version.
- Add the Plesk tools' issue trackers and logs to the issue-reporting list.
- Troubleshooting: CageFS users not entering CageFS through su after the
  upgrade (CLOS-7025), with the cagefsctl --hook-install workaround.
- Known issue: MySQL Governor with cl-MariaDB103 on Plesk - switch to
  MariaDB 10.6 before upgrading.
- Fix links to the no-panel scenario, broken since the heading gained
  "(incl. DirectAdmin)".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Worked-On: cl-aiworkspaces