fix(setupChecks): Update Transactional File Locking instructions

[joshtrichards]

Summary

The filelocking.enabled toggle is no longer relevant today since TFL is now on by default and is no longer an experimental feature (hasn't been for nearly a decade). There is no good reason to disable it. The toggle only exists to be able to enable it, when it was still experimental. It's also been removed from the docs.

This PR:

• makes the setup check firmer
• enhances the instructions
• changes from a warning to an error
• renames check from "File Locking" -> "Transaction File Locking" for consistency with docs (and to distinguish from types)

Complements #45330 and nextcloud/documentation#11848 (does not depend on either)

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)