Docs: replace deprecated http config directives with xrd ones

See https://xrootd.slac.stanford.edu/doc/dev57/xrd_config.htm#_Toc171719990 which says:

The xrd framework provides TLS services to all protocols and HTTPS relies on these services. The xrd.tls and xrd.tlsca directives should be used to configure TLS for HTTPS. Previous releases relied on http specific directives to do this (i.e. http.cadir, http.cafile, http.cert and http.key). These are still accepted and still allow HTTPS to be configured. However, when specified a warning message is issued to remind you to use the xrd framework directives instead. It is best to configure TLS for all protocols using a common set of directives to avoid inconsistency. Therefore, the http-specific TLS directives have been deprecated.