Produce at least something for unknown moderation log messages

The reason for this is some people might be migrating back from
Rebased or even Akkoma, which may have their own moderation log
messages that pleroma does not support. We should at least produce
something instead of erroring out.

changelog.d/unknown-mod-log.fix

@@ -0,0 +1 @@
+Produce "unknown message" instead of an error when the moderation log message is not recognized

lib/pleroma/moderation_log.ex

@@ -575,6 +575,10 @@ defmodule Pleroma.ModerationLog do
     "@#{actor_nickname} requested account backup for @#{user_nickname}"
   end
 
+  def get_log_entry_message(%ModerationLog{} = data) do
+    "Unknown log entry: #{inspect(data)}"
+  end
+
   defp nicknames_to_string(nicknames) do
     nicknames
     |> Enum.map(&"@#{&1}")