Check conformance classes

[m-mohr]

It seems there are various inconsistencies in the conformance classes.

For example, I see http://www.opengis.net/spec/ogcapi-features-3/1.0/req/features-filter (in https://portal.ogc.org/files/96288#features-filter-overview) and http://www.opengis.net/spec/ogcapi-features-3/1.0/conf/features-filter (in this repo). Which is correct?

Then there seems to be a copy/paste error for ogcapi-features classes: see #312

This conformance classes should closely checked as this is pretty crucial in implementations and already leads to a lot of issues in implementations (see STAC Fast API issues or the STAC Browser implementation)

[philvarner]

"conf" is correct per for features per https://github.com/opengeospatial/ogcapi-features/blob/master/extensions/filtering/standard/clause_2_conformance.adoc

"conf" is the conformance class, "req" is the requirements class (honestly, I'm still unclear what the difference is, but we only use the conformance classes)