src/external: Update to Catch2 version 2.13.7

Required to fix build on Ubuntu Impish (and newer, presumably). The version we're using is ancient (pre-buster).

I'll need to update the Impish package when this gets merged, because right now it's being applied as a patch there.

@haagch I think you mentioned hitting this issue too? I tried doing the debian maintainer thing and just minimally backporting the fix for Catch2 but that didn't work, nor did using the system catch2 (had really weird errors in that case).