Add filter to specification-series by superseded status

[dontcallmedom]

If the last version of a spec in a series is supersed (e.g. https://api.w3.org/specification-series/ATAG-wombat), it would be good to be able to have a parameter on the specification-series endpoint that exclude the said series.

[dontcallmedom]

cc @deniak

[dontcallmedom]

(another approach might to remove the said series by merging the shortname with whatever supersedes them)

[deniak]

We are already giving the possibility to filter out the retired specifications so we can add an extra parameter to do the same for series with superseded specifications. I've submitted a PR and waiting for someone to review it.