devel/catch: update to 2.3.0

This release changes the include paths provided by our CMake and pkg-config
integration. The proper include path for the single-header when using one of
the above is now <catch2/catch.hpp>. This change also necessitated changes to
paths inside the repository, so that the single-header version is now at
single_include/catch2/catch.hpp, rather than single_include/catch.hpp.

Release notes: https://github.com/catchorg/Catch2/releases/tag/v2.3.0