libxmlsec1 1.2.20

Version bump. I’ve also moved from GnuTLS to OpenSSL, because even when configured explicitly with GnuTLS it was still pulling in the system OpenSSL and using that in part anyway, and having both crypto libraries as dependencies just seemed redundant when libxmlsec1 allows you to use either. If anyone strongly objects I guess I’m open to having both libraries as dependencies.

Please don't merge this before #33288.