feat: Delegate is-encrypted handling to the e2ee app

[artonge]

The is-encrypted property is signaling the E2EE status, not the SSE one. So it does not make sense to keep it in the server repo.

• E2EE counterpart: feat: Own the handling of the is-encrypted DAV property end_to_end_encryption#733

[artonge]

/backport to stable30

[come-nc]

Wait, but FileInfo::isEncrypted is used for SSE and not E2EE, no?

[artonge]

Wait, but FileInfo::isEncrypted is used for SSE and not E2EE, no?

Not sure that I get the concern, but in this case, isEncrypted() was called inside a $node instanceof Directory condition, so this was only targeting E2EE folders and not SSE files.