Deprecate unused last_modified attribute

[tlater-famedly]

We do not use the last_modified attribute defined here for anything anymore: https://github.com/famedly/famedly-sync/blob/main/src/sources/ldap.rs#L319

Its purpose was to tell ldap_poller which changes should be considered, since it fundamentally relied on processing the user list over time, however we have switched to a full sync algorithm as that is more robust to partial/interrupted syncs. As such, it no longer serves a purpose, and we shouldn't force users to define it.

This will be a breaking change, though we can simply ignore the attribute when it exists, and otherwise remove it from all our documentation.