Add dbf data format specifier to older FERC files

[zaneselvans]

Right now the earlier files in the FERC archives don't have any indication as to what filetype they contain, but the newer ones include xbrl. E.g. ferc1-2021.zip (which contains DBF data) vs. ferc1-xbrl-2021.zip

This inconsistency is a little confusing especially as we're accumulating updates to older data which is published in XBRL now, and probably introduces some unnecessary complexity in parsing the archives. By the same token as we've renamed our output databases ferc1_dbf.sqlite and ferc1_xbrl.sqlite it seems like it would make sense to rename the raw data archives.