Handle captrunc and emptycapped during tenant migrations

captrunc truncates documents from a capped collection. It is a testing-only command that does no auth checks and doesn't generate oplog entries.

emptycapped deletes all documents from a capped collection. It is a testing-only command that does no auth checks, though it does generate oplog entries.

Since these are testing-only commands, I don't think we need to support them in serverless.

Note that patch set 1 of this CR for SERVER-49174 contains the code for handling captrunc but it was decided that it should be tackled in a separate ticket.