Set attribute `deny_unknown_fields` for Lemmy config (#2852)

Set attribute `deny_unknown_fields` for Lemmy config (#2852)

With this attribute, Lemmy will throw an error and exit if any
invalid entry is found in the config file. I think can be useful
to notice typos or keys that were removed or renamed in an upgrade.
Currently you wouldnt notice these at all unless you manually
compare the config file with settings that are listed in documentation.

This should be considered a breaking change.

Co-authored-by: Dessalines <dessalines@users.noreply.github.com>